This opens more possibilities, like accessing context.template.origin.
It also follows the chain of objects instead of following a shortcut.
Backport of 1bfcc95 from master
A deprecation path is required because the return type of
django.template.loader.get_template changed during the
multiple template engines refactor.
test_csrf_token_in_404 was incorrect: it tested the case when the
hardcoded template was rendered, and that template doesn't depend on the
CSRF token. This commit makes it test the case when a custom template is
rendered.
