Custom emoji (#4988)

* 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

app/models/status.rb

@@ -131,6 +131,10 @@ class Status < ApplicationRecord
     !sensitive? && media_attachments.any?
   end
 
+  def emojis
+    CustomEmoji.from_text(text, account.domain)
+  end
+
   after_create :store_uri, if: :local?
 
   before_validation :prepare_contents, if: :local?