Allow Mastodon address domain to be different from primary domain

app/models/setting.rb

@@ -106,6 +106,9 @@ class Setting < RailsSettings::Base
   field :mastodon_enabled, type: :boolean,
     default: (ENV["MASTODON_PUBLIC_URL"].present?.to_s || false)
 
+  field :mastodon_address_domain, type: :string,
+    default: ENV["MASTODON_ADDRESS_DOMAIN"].presence || self.primary_domain
+
   #
   # MediaWiki
   #