* Custom emoji
- In OStatus: `<link rel="emoji" name="coolcat" href="http://..." />`
- In ActivityPub: `{ type: "Emoji", name: ":coolcat:", href: "http://..." }`
- In REST API: Status object includes `emojis` array (`shortcode`, `url`)
- Domain blocks with reject media stop emojis
- Emoji file up to 50KB
- Web UI handles custom emojis
- Static pages render custom emojis as `<img />` tags
Side effects:
- Undo #4500 optimization, as I needed to modify it to restore
shortcode handling in emojify()
- Formatter#plaintext should now make sure stripped out line-breaks
and paragraphs are replaced with newlines
* Fix emoji at the start not being converted
