Encode custom emojis as resolveable objects in ActivityPub (#5243)

* Encode custom emojis as resolveable objects in ActivityPub

* Improve code style

app/models/custom_emoji.rb

@@ -13,6 +13,8 @@
 #  created_at         :datetime         not null
 #  updated_at         :datetime         not null
 #  disabled           :boolean          default(FALSE), not null
+#  uri                :string
+#  image_remote_url   :string
 #
 
 class CustomEmoji < ApplicationRecord
@@ -37,6 +39,10 @@ class CustomEmoji < ApplicationRecord
     domain.nil?
   end
 
+  def object_type
+    :emoji
+  end
+
   class << self
     def from_text(text, domain)
       return [] if text.blank?