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/lib/activitypub/activity/create.rb

@@ -61,6 +61,8 @@ class ActivityPub::Activity::Create < ActivityPub::Activity
         process_hashtag tag, status
       when 'Mention'
         process_mention tag, status
+      when 'Emoji'
+        process_emoji tag, status
       end
     end
   end
@@ -79,6 +81,17 @@ class ActivityPub::Activity::Create < ActivityPub::Activity
     account.mentions.create(status: status)
   end
 
+  def process_emoji(tag, _status)
+    shortcode = tag['name'].delete(':')
+    emoji     = CustomEmoji.find_by(shortcode: shortcode, domain: @account.domain)
+
+    return if !emoji.nil? || skip_download?
+
+    emoji = CustomEmoji.new(domain: @account.domain, shortcode: shortcode)
+    emoji.image_remote_url = tag['href']
+    emoji.save
+  end
+
   def process_attachments(status)
     return unless @object['attachment'].is_a?(Array)