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/views/settings/_xmpp.html.erb

@@ -3,8 +3,8 @@
   <h3>Contacts</h3>
   <ul role="list">
     <%= render FormElements::FieldsetToggleComponent.new(
-      field_name: "user[preferences][xmpp][exchange_contacts_with_invitees]",
-      enabled: @user.pref_enabled?("xmpp:exchange_contacts_with_invitees"),
+      field_name: "user[preferences][xmpp_exchange_contacts_with_invitees]",
+      enabled: @user.preferences[:xmpp_exchange_contacts_with_invitees],
       title: "Exchange contacts when invited user signs up",
       description: "Add each others contacts, so you can chat with them immediately"
     ) %>