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

* Encode custom emojis as resolveable objects in ActivityPub

* Improve code style

app/serializers/activitypub/image_serializer.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+class ActivityPub::ImageSerializer < ActiveModel::Serializer
+  include RoutingHelper
+
+  attributes :type, :media_type, :url
+
+  def type
+    'Image'
+  end
+
+  def url
+    full_asset_url(object.url(:original))
+  end
+
+  def media_type
+    object.content_type
+  end
+end