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/webhooks_controller.rb

@@ -12,7 +12,7 @@ class WebhooksController < ApplicationController
     end
 
     user = User.find_by!(ln_account: payload[:user_login])
-    notify = user.preferences.dig("lightning", "notify_sats_received")
+    notify = user.preferences[:lightning_notify_sats_received]
     case notify
     when "xmpp"
       notify_xmpp(user.address, payload[:amount], payload[:memo])