We currently have a bug where a model's relation tree can be cached before

an extension providing a related model is registered, leaving the relation

permanently missing from it.
Django's Apps.set_installed_apps() sets Apps.ready to False before both

of its calls to Apps.clear_cache() (its own, and the one at the end of

Apps.populate()). However clear_cache() only expires the per-model

Options caches when the registry is ready. Installing an app therefore

never rebuilds those cached relation trees.
This bug is exposed when an extension imports something from another extension

at module scope. For example, an extension that uses Power Pack's User Roles

in an approval hook will likely import the user role model at the module scope

(we do this in an example in Review Board's documentation). The extension

manager imports every extension's module before initializing any of them, so

in this case the related model gets registered but is unreachable through

AppConfig. Then if something accesses the model's relation tree before the

extensions are registered, the tree is cached without the extension model's

relation. This was bug was discovered when trying to import User Roles in

RBCommons' team admin code.
This change makes ExtensionManager._add_to_installed_apps() clear the cache

itself, so that model relations can be properly populated.

Ran unit tests.
Saw the FieldError I was getting in RBCommons from the User Roles

  relation being missing go away.