Deprecated passing a Context to a generic Template.render.

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.