reviewboard.integrations provided convenience imports for

get_integration_manager() and the base Integration class. These

ended up being a bad idea, because the imports needed for these ended up

interfering with Django app initialization.
Now, both of these forward on to the versions in

reviewboard.integrations.base, showing a deprecation warning on use.

This will remain for Review Board 4.0, and are planned to be removed in

5.0.
To handle the deprecation of the Integration class, we use a metaclass

that will handle the construction of any subclasses. It will reassign

the parent to reviewboard.integrations.base.Integration. This

shouldn't break any subclasses, but could cause problems if anything is

trying to directly compare against the version in

reviewboard.integrations. As this is unlikely, and fixing this would

complicate things quite a bit, we won't be addressing it unless a

problem comes up somewhere.

Unit tests pass on Django 1.6 and 1.11 (with other in-progress changes).
Verified that older integrations were loading correctly on both versions

of Django.