Refactor settings model

Move the various sections to their own concerns, so they're easier to
find and maintain

app/models/concerns/settings/xmpp_settings.rb

@@ -0,0 +1,11 @@
+module Settings
+  module XmppSettings
+    extend ActiveSupport::Concern
+
+    included do
+      field :xmpp_default_rooms, type: :array, default: []
+      field :xmpp_autojoin_default_rooms, type: :boolean, default: false
+      field :xmpp_notifications_from_address, type: :string, default: primary_domain
+    end
+  end
+end