Fixed #15053 -- Improved template extending algorithm.
Updated the debug views.

Since the list of tried templates are now part of TemplateDoesNotExist,
the logic here can be much simpler. This has the nice side-effect of fixing
the template post-mortem for the egg and cached loaders.
Added deprecation warning for old loader api usage.
Catch recursion with old-style template loaders up-front.
Added documentation for the new loader api.