Django caches a bunch of field information so when we side load our

extension Models any relationships added will not be propogated to the

related Model. This will cause problems where something tries to access

the related name of the extension Model from the built-in Model.
We now flush the caches on the related built-in Models when enabling

an extension.

Tested using the AutomaticRunGroup Model in Review Bot which relates

to the Repository Model. The admin interface for changing the

AutomaticRunGroup entry now loads properly.