ct_tracker: Do not call invalidate_cache if the base model does not have it
Make urlconf arg of app_reverse a kwarg, so that one can curry the app_reverse into a reverse instead of dribbling changes all over the external application.

This should make it possible to do

    reverse = functool.partial(app_reverse, urlconf='foo')

at the top of the app once instead of changing all reverses into app_reverses.
Merge branch 'next' of github.com:mjl/feincms into next
Merge branch 'master' into next

* master:
  ct_tracker: Do not call invalidate_cache if the base model does not have it