Add short_description and approval_required to GET /api/v1/instance (mastodon#11146)

Author: Gargron

app/serializers/rest/instance_serializer.rb

@@ -3,9 +3,9 @@
 class REST::InstanceSerializer < ActiveModel::Serializer
   include RoutingHelper
 
-  attributes :uri, :title, :description, :email,
+  attributes :uri, :title, :short_description, :description, :email,
              :version, :urls, :stats, :thumbnail,
-             :languages, :registrations
+             :languages, :registrations, :approval_required
 
   has_one :contact_account, serializer: REST::AccountSerializer
 
@@ -19,6 +19,10 @@ def title
     Setting.site_title
   end
 
+  def short_description
+    Setting.site_short_description
+  end
+
   def description
     Setting.site_description
   end
@@ -55,6 +59,10 @@ def registrations
     Setting.registrations_mode != 'none' && !Rails.configuration.x.single_user_mode
   end
 
+  def approval_required
+    Setting.registrations_mode == 'approved'
+  end
+
   private
 
   def instance_presenter