[1.8.x] Fixed #24168 -- Allowed selecting a template engine in a few APIs.

Specifically in rendering shortcuts, template responses, and class-based
views that return template responses.

Also added a test for render_to_response(status=...) which was missing
from fdbfc980.

Thanks Tim and Carl for the review.

Backport of 2133f315 from master.

Conflicts:
	docs/topics/http/shortcuts.txt
	tests/generic_views/test_base.py