Switch to the new template loaders from Djblets.

This updates our list of template loaders to use the new
conditional_cached (which only caches if DEBUG = False) and
namespaced_app_dirs (which allows app module prefixes for paths).
Add a link to the package store on the extensions page.

This adds a "Get more extensions" link on the extensions page, to the
left of the "Scan for installed extensions" link. This link points to
the URL for the upcoming Package Store.

Reviewed at https://reviews.reviewboard.org/r/6093/