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).

Reviewed at https://reviews.reviewboard.org/r/6092/
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/
Merge branch 'release-2.0.x'
Refactor $.fn.bug_infobox and $.fn.user_infobox to share code.

This is the first step in refactoring the infoboxes to use backbone models and
views. The code for user and bug infoboxes was substantially similar, and
there's no reason why they had to be separate. All of the code is now shared,
and the API entry-points (`bug_infobox` and `user_infobox`) just call into the
common method and pass an ID for the infobox element.

Testing done:
Tested that both infoboxes worked as expected.

Reviewed at https://reviews.reviewboard.org/r/6053/