Work around Django form util rename

Review Request #7608 — Created Aug. 25, 2015 and submitted — Latest diff uploaded

Information

Djblets
master

Reviewers

Django has renamed django.form.util to django.form.utils in 1.7+.
This attempts to import both versions where it is required. The
previous patch that attempted to fix this issue did not update imports
of django.form.util.