Add account hashtags to ActivityPub actor JSON (mastodon#9450)

app/serializers/activitypub/actor_serializer.rb

@@ -109,7 +109,7 @@ def is_cat
   end
 
   def virtual_tags
-    object.emojis
+    object.emojis + object.tags
   end
 
   def virtual_attachments
@@ -123,6 +123,24 @@ def moved_to
   class CustomEmojiSerializer < ActivityPub::EmojiSerializer
   end
 
+  class TagSerializer < ActiveModel::Serializer
+    include RoutingHelper
+
+    attributes :type, :href, :name
+
+    def type
+      'Hashtag'
+    end
+
+    def href
+      explore_hashtag_url(object)
+    end
+
+    def name
+      "##{object.name}"
+    end
+  end
+
   class Account::FieldSerializer < ActiveModel::Serializer
     attributes :type, :name, :value