Fix a regression in clearing the app cache on extension initialization.

Review Request #8927 — Created May 8, 2017 and submitted — Latest diff uploaded

The work done to improve extension management, loading, and Django
compatibility in Djblets 0.10 regressed clearing the app cache state for
Django 1.6. We were only resetting the app cache's loaded flag if
attempting to update the database, which wasn't enough. Amongst other
possible regressions, this prevented evolutions from being able to be

The clearing has now been moved out of the database upgrade code and
into the methods for manipulating the list of installed apps.

Verified in a shell that the extensions were listed as part of a
get_apps() call, which wasn't the case before.

Deployed this to, and verified that it was
including our extensions when considering database upgrades.