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/models/user.rb

@@ -1,7 +1,7 @@
 class User < ApplicationRecord
   include EmailValidatable
 
-  serialize :preferences, Hash, default: {}
+  serialize :preferences, UserPreferences
 
   # Relations
   has_many :invitations, dependent: :destroy
@@ -64,7 +64,7 @@ class User < ApplicationRecord
 
     if inviter.present?
       if Setting.ejabberd_enabled? &&
-          inviter.pref_enabled?("xmpp:exchange_contacts_with_invitees")
+          inviter.preferences[:xmpp_exchange_contacts_with_invitees]
         exchange_xmpp_contact_with_inviter
       end
     end
@@ -138,11 +138,6 @@ class User < ApplicationRecord
     ldap.delete_attribute(dn,:service)
   end
 
-  def pref_enabled?(key)
-    value = preferences.dig(*key.split(":"))
-    [true, "true", "enabled", 1].include?(value)
-  end
-
   def exchange_xmpp_contact_with_inviter
     return unless inviter.services_enabled.include?("xmpp") &&
                   services_enabled.include?("xmpp")