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

The list of tried templates is now provided as part of the
TemplateDoesNotExist error. This means we no longer need to recalculated it.
This new method means the egg and cached loaders will now also appear in the
template post-mortem.
Added deprecation warning for old loader api usage.
Catch recursion with old-style template loaders up-front.
Added documentation for the new loader api.