Refactor user preferences, add defaults from file

* Turn prefs into a flat hash structure, since nesting is not worth the
trouble
* Add a custom serializer class for prefs
* Add a config file for defaults and merge set prefs with unset ones
* Use booleans for "true" and "false", and integers where appropriate

app/controllers/settings_controller.rb

@@ -45,8 +45,8 @@ class SettingsController < ApplicationController
 
   def user_params
     params.require(:user).permit(preferences: [
-      lightning: [:notify_sats_received],
-      xmpp: [:exchange_contacts_with_invitees]
+      :lightning_notify_sats_received,
+      :xmpp_exchange_contacts_with_invitees
     ])
   end
 end