Add basic support for group actors (#12071)

* Show badge on group actor in WebUI

* Do not notify in case of  by following group actor

* If you mention group actor, also mention group actor followers

* Relax characters that can be used in username (same as Application)

* Revert "Relax characters that can be used in username (same as Application)"

This reverts commit 7e10a137b878d0db1b5252c52106faef5e09ca4b.

* Delete display_name method

app/serializers/activitypub/actor_serializer.rb

@@ -49,6 +49,8 @@ class ActivityPub::ActorSerializer < ActivityPub::Serializer
       'Application'
     elsif object.bot?
       'Service'
+    elsif object.group?
+      'Group'
     else
       'Person'
     end

app/serializers/rest/account_serializer.rb

@@ -3,7 +3,7 @@
 class REST::AccountSerializer < ActiveModel::Serializer
   include RoutingHelper
 
-  attributes :id, :username, :acct, :display_name, :locked, :bot, :discoverable, :created_at,
+  attributes :id, :username, :acct, :display_name, :locked, :bot, :discoverable, :group, :created_at,
              :note, :url, :avatar, :avatar_static, :header, :header_static,
              :followers_count, :following_count, :statuses_count, :last_status_at