Reduce run-time introspection of the apps module

[JustinTervala]

Currently the controller introspects the apps module to find the correct app to construct at run time. We could reduce this by making the apps.App class inherit a metaclass which has a registry. A similar approach has been used in flags and filters, and now that the apps.App class does not inherit the SQLAlchemy metaclass, we don't need to deal with the issue of metaclass multiple inheritance. This new architecture would further reduce dependencies between the apps and the controller modules.