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

spec/requests/webhooks_spec.rb

@@ -68,7 +68,7 @@ RSpec.describe "Webhooks", type: :request do
 
       context "notification preference set to 'xmpp'" do
         before do
-          user.update! preferences: { "lightning" => { "notify_sats_received" => "xmpp" }}
+          user.update! preferences: { lightning_notify_sats_received: "xmpp" }
           post "/webhooks/lndhub", params: payload.to_json
         end
 
@@ -87,7 +87,7 @@ RSpec.describe "Webhooks", type: :request do
 
       context "notification preference set to 'email'" do
         before do
-          user.update! preferences: { "lightning" => { "notify_sats_received" => "email" }}
+          user.update! preferences: { lightning_notify_sats_received: "email" }
           post "/webhooks/lndhub", params: payload.to_json
         end