Add support for links to statuses in announcements to be opened in web UI (#13212)

* Add support for links to public statuses in announcements to be opened in WebUI

* Please CodeClimate

app/serializers/rest/announcement_serializer.rb

@@ -7,6 +7,7 @@ class REST::AnnouncementSerializer < ActiveModel::Serializer
   attribute :read, if: :current_user?
 
   has_many :mentions
+  has_many :statuses
   has_many :tags, serializer: REST::StatusSerializer::TagSerializer
   has_many :emojis, serializer: REST::CustomEmojiSerializer
   has_many :reactions, serializer: REST::ReactionSerializer
@@ -46,4 +47,16 @@ class REST::AnnouncementSerializer < ActiveModel::Serializer
       object.pretty_acct
     end
   end
+
+  class StatusSerializer < ActiveModel::Serializer
+    attributes :id, :url
+
+    def id
+      object.id.to_s
+    end
+
+    def url
+      ActivityPub::TagManager.instance.url_for(object)
+    end
+  end
 end