Removed the only_installed argument of get_app_config[s].

It wasn't used anywhere and couldn't be implemented any more since
non-installed apps no longer have a configuration.
Removed the app_config.installed flag.

Since applications that aren't installed don't have an application
configuration, it would always be true in practice.

Provided an abstraction to temporarily add or remove an application as
several tests messed with app_config.installed to achieve this effect.
For now this API is _-prefixed because it looks dangerous.
Implemented two-stage app-cache population.

First stage imports app modules. It doesn't catch import errors. This
matches the previous behavior and keeps the code simple.

Second stage import models modules. It catches import errors and retries
them after walking through the entire list once. This matches the
previous behavior and seems useful.

populate_models() is intended to be equivalent to populate().