Add new template loaders for conditional caching and namespaced apps.

This adds two new template loaders intended to directly replace standard
Django template ones.

The new 'conditional_cached' template loader functions like Django's
'cached' loader, but does not cache if DEBUG is True. This helps a lot
with development, since it's no longer necessary to reload the server
after modifying a template.

The new 'namespaced_app_dirs' template loader is like Django's
'app_directories' loader, but accepts an optional namespace prefix in
the form of 'app.path:'. If specified, the list of template directories
will be limited to those owned by the given app path.

Reviewed at https://reviews.reviewboard.org/r/6070/