Move hard-coded settings to proper user prefs

Here are the settings to put in build/config/custom-prefs.js:

  user_pref("services.fxaccounts.contacts.url", "http://localhost/owncloud/remote.php/carddav/addressbooks/me/contacts");
  user_pref("services.fxaccounts.contacts.username", "me");
  user_pref("services.fxaccounts.contacts.password", "Password1!");