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. Testing Done: Ran tests in both Djblets and Review Board (the latter being where I was hitting these issues).