Ensure compatibility patches are only applied once.
Ensure compatibility patches are only applied once.

`apply_patches()` can technically be called more than once, which could
result in some bad patches. As a precaution, it now checks if it's been
called before, and if so, it bails.

Testing Done:
Unit tests pass.

Reviewed at https://reviews.reviewboard.org/r/11899/

beanbaginc/django-evolution