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

* Encode custom emojis as resolveable objects in ActivityPub

* Improve code style

app/serializers/activitypub/emoji_serializer.rb

@@ -0,0 +1,29 @@
+# frozen_string_literal: true
+
+class ActivityPub::EmojiSerializer < ActiveModel::Serializer
+  include RoutingHelper
+
+  attributes :id, :type, :name, :updated
+
+  has_one :icon, serializer: ActivityPub::ImageSerializer
+
+  def id
+    ActivityPub::TagManager.instance.uri_for(object)
+  end
+
+  def type
+    'Emoji'
+  end
+
+  def icon
+    object.image
+  end
+
+  def updated
+    object.updated_at.iso8601
+  end
+
+  def name
+    ":#{object.shortcode}:"
+  end
+end