Fix an HTTP 500 error when user registration fails.

A backport of some code to the 0.7.x releases resulted in user
registration failures being handled incorrectly. This was just due to a
couple missing imports. These have now been added, and old unused
imports have been removed.
Release Djblets 0.7.28.
Specify TRACKING_BRANCH in release-0.7.x's .reviewboardrc
Merge branch 'release-0.7.x'