Turn default services into a configurable setting

With the default value being all enabled services

app/models/setting.rb

@@ -21,8 +21,14 @@ class Setting < RailsSettings::Base
   include Settings::RemoteStorageSettings
   include Settings::XmppSettings
 
-  def self.default_services
-    # TODO Make configurable from respective service settings page
-    %w[ discourse gitea mastodon mediawiki remotestorage xmpp ]
+  def self.available_services
+    known_services = %w[
+      discourse ejabberd email gitea mastodon mediawiki remotestorage
+    ]
+
+    known_services.select {|s| Setting.send "#{s}_enabled?" }
   end
+
+  field :default_services, type: :array,
+    default: self.available_services
 end