Avoid fetching each user by passing in full pre-fetched
presences from Presence.fetch/2 callback.
Use temporary assigns in ProfileLive to avoid duping
presences in memeory.
Handle removes by a small hook event
* Use `HtmlElement.tabIndex` directly rather than going through attributes.
* Always restore `tabIndex` after focus.
* Remove `setTimeout` to simplify implementation.
This does have a disadvantage in that it leaves unnecessary `tabindex="1"` instances around. On the other hand, it does simplify the implementation and get the job done' so is probably a more clear example of how to do this correctly.
* Add handler for `phx:page-loading-stop` that focuses either the first `<h1>` child of `<main>`, or `<main>` directly if no child `<h1>` is present. Make this our focus target.
* Cache original `tabindex` of target, if any, and assign it a temporary `tabindex1 of -1.
* Focus the target.
* After a long timeout, either restore the target's original `tabindex` or remove the temporary -1 value. Short timeouts didn't seem to perform the focus step. Neither did nested `requestAnimationFrame` calls.
