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/serializers/activitypub/note_serializer.rb

@@ -57,7 +57,7 @@ class ActivityPub::NoteSerializer < ActiveModel::Serializer
   end
 
   def virtual_tags
-    object.mentions + object.tags
+    object.mentions + object.tags + object.emojis
   end
 
   def atom_uri
@@ -137,4 +137,22 @@ class ActivityPub::NoteSerializer < ActiveModel::Serializer
       "##{object.name}"
     end
   end
+
+  class CustomEmojiSerializer < ActiveModel::Serializer
+    include RoutingHelper
+
+    attributes :type, :href, :name
+
+    def type
+      'Emoji'
+    end
+
+    def href
+      full_asset_url(object.image.url)
+    end
+
+    def name
+      ":#{object.shortcode}:"
+    end
+  end
 end