In the case of unit tests, we may be rendering some templates that

include {% template_hook_point %} without going through all our usual

context processor machinery. In this case, we can sometimes hit cases

where we're calling this tag with a RequestContext that does not

actually have request in the dict contents. This was causing some unit

test failures.
This change makes it so our template tag will check if we have a

RequestContext, and if so, get the request from the attribute instead

of trying to pull it out of the dict. This fixes the cases where unit

tests were crashing.

Ran tests in both Djblets and Review Board (the latter being where I

was hitting these issues).