generate user registration form

http://manage.dimagi.com/default.asp?122848

`app.get_user_registration()` gets called in `app.wrap` (via `app.get_forms()`) which sets the value of `user_registration.unique_id` but does not save the app.

Rather than load the form and set it's unique_id on request, this change does that as soon as the user requests a custom registration form i.e. when `app.show_user_registration` is set to `True`.