Fix a regression in clearing the app cache on extension initialization.
Review Request #8927 - Created May 8, 2017 and submitted
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 updated. 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 reviews.reviewboard.org, and verified that it was
including our extensions when considering database upgrades.