Creates an OpenGraph template include in base.html including the basic tags expected on all pages.
Then allows any page to add additional expected tags via `context`.
Currently, profiles and posts are enriched to show complete opengraph metadata, and render correctly in Discord.
Note: This does not show posts in Slack like Twitter/Mastodon do. I believe this is due to Slack preferring oembed when present, which is a mastodon API endpoint we may need to create at some point.
