Docs: Add chat motivation and api description.

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# Chats
Chats are a way to represent an IM-style conversation between two actors. They are not the same as direct messages and they are not `Status`es, even though they have a lot in common.
## Why Chats?
There are no 'visibility levels' in ActivityPub, their definition is purely a Mastodon convention. Direct Messaging between users on the fediverse has mostly been modeled by using ActivityPub addressing following Mastodon conventions on normal `Note` objects. In this case, a 'direct message' would be a message that has no followers addressed and also does not address the special public actor, but just the recipients in the `to` field. It would still be a `Note` and is presented with other `Note`s as a `Status` in the API.
This is an awkward setup for a few reasons:
- As DMs generally still follow the usual `Status` conventions, it is easy to accidentally pull somebody into a DM thread by mentioning them. (e.g. "I hate @badguy so much")
- It is possible to go from a publicly addressed `Status` to a DM reply, back to public, then to a 'followers only' reply, and so on. This can be become very confusing, as it is unclear which user can see which part of the conversation.
- The standard `Status` format of implicit addressing also leads to rather ugly results if you try to display the messages as a chat, because all the recipients are always mentioned by name in the message.
- As direct messages are posted with the same api call (and usually same frontend component) as public messages, accidentally making a public message private or vice versa can happen easily. Client bugs can also lead to this, accidentally making private messages public.
As a measure to improve this situation, the `Conversation` concept and related Pleroma extensions were introduced. While it made it possible to work around a few of the issues, many of the problems remained and it didn't see much adoption because it was too complicated to use correctly.
## Chats explained
For this reasons, Chats are a new and different entity, both in the API as well as in ActivityPub. A quick overview:
- Chats are meant to represent an instant message conversation between two actors. For now these are only 1-on-1 conversations, but the other actor can be a group in the future.
- Chat messages have the ActivityPub type `ChatMessage`. They are not `Note`s. Servers that don't understand them will just drop them.
- The only addressing allowed in `ChatMessage`s is one single ActivityPub actor in the `to` field.
- There's always only one Chat between two actors. If you start chatting with someone and later start a 'new' Chat, the old Chat will be continued.
- `ChatMessage`s are posted with a different api, making it very hard to accidentally send a message to the wrong person.
- `ChatMessage`s don't show up in the existing timelines.
- Chats can never go from private to public. They are always private between the two actors.
## Caveats
- Chats are NOT E2E encrypted (yet). Security is still the same as email.
## API
In general, the way to send a `ChatMessage` is to first create a `Chat`, then post a message to that `Chat`. The actors in the API are generally given by their ActivityPub id to make it easier to support later `Group` scenarios.
This is the overview of using the API. The API is also documented via OpenAPI, so you can view it and play with it by pointing SwaggerUI or a similar OpenAPI tool to `https://yourinstance.tld/api/openapi`.
### Creating or getting a chat.
To create or get an existing Chat for a certain recipient (identified by AP ID)
you can call:
`POST /api/v1/pleroma/chats/by-ap-id/{ap_id}`
The ap_id of the recipients needs to be www-form encoded, so
```
https://originalpatchou.li/user/lambda
```
would become
```
https%3A%2F%2Foriginalpatchou.li%2Fuser%2Flambda
```
The full call would then be
```
POST /api/v1/pleroma/chats/by-ap-id/https%3A%2F%2Foriginalpatchou.li%2Fuser%2Flambda
```
There will only ever be ONE Chat for you and a given recipient, so this call
will return the same Chat if you already have one with that user.
Returned data:
```json
{
"recipient" : "https://dontbulling.me/users/lain",
"recipient_account": {
"id": "someflakeid",
"username": "somenick",
...
},
"id" : "1",
"unread" : 2
}
```
### Getting a list of Chats
`GET /api/v1/pleroma/chats`
This will return a list of chats that you have been involved in, sorted by their
last update (so new chats will be at the top).
Returned data:
```json
[
{
"recipient" : "https://dontbulling.me/users/lain",
"recipient_account": {
"id": "someflakeid",
"username": "somenick",
...
},
"id" : "1",
"unread" : 2
}
]
```
The recipient of messages that are sent to this chat is given by their AP ID.
The usual pagination options are implemented.
### Getting the messages for a Chat
For a given Chat id, you can get the associated messages with
`GET /api/v1/pleroma/chats/{id}/messages`
This will return all messages, sorted by most recent to least recent. The usual
pagination options are implemented
Returned data:
```json
[
{
"actor": "https://dontbulling.me/users/lain",
"chat_id": "1",
"content": "Check this out :firefox:",
"created_at": "2020-04-21T15:11:46.000Z",
"emojis": [
{
"shortcode": "firefox",
"static_url": "https://dontbulling.me/emoji/Firefox.gif",
"url": "https://dontbulling.me/emoji/Firefox.gif",
"visible_in_picker": false
}
],
"id": "13"
},
{
"actor": "https://dontbulling.me/users/lain",
"chat_id": "1",
"content": "Whats' up?",
"created_at": "2020-04-21T15:06:45.000Z",
"emojis": [],
"id": "12"
}
]
```
### Posting a chat message
Posting a chat message for given Chat id works like this:
`POST /api/v1/pleroma/chats/{id}/messages`
Parameters:
- content: The text content of the message
Currently, no formatting beyond basic escaping and emoji is implemented, as well as no
attachments. This will most probably change.
Returned data:
```json
{
"actor": "https://dontbulling.me/users/lain",
"chat_id": "1",
"content": "Check this out :firefox:",
"created_at": "2020-04-21T15:11:46.000Z",
"emojis": [
{
"shortcode": "firefox",
"static_url": "https://dontbulling.me/emoji/Firefox.gif",
"url": "https://dontbulling.me/emoji/Firefox.gif",
"visible_in_picker": false
}
],
"id": "13"
}
```
### Notifications
There's a new `pleroma:chat_mention` notification, which has this form:
```json
{
"id": "someid",
"type": "pleroma:chat_mention",
"account": { ... } // User account of the sender,
"chat_message": {
"chat_id": "1",
"id": "10",
"content": "Hello",
"actor": "https://dontbulling.me/users/lain"
},
"created_at": "somedate"
}
```