Fix #121 - Generate form success key from ident, not pk
Fix #121 - Generate form success key from ident, not pk

The logical form has a new pk for each version, which means the success
page changed on every commit. Now it uses a key generated from its
ident, which never changes. This allows caching form pages because the
success urls for all versions of the form are the same.

j00bar/django-widgy,j00bar/django-widgy,j00bar/django-widgy