update mitxmako lookup / fixes FOR-150

@cpennington @kevinchugh

this fixes the underlying AttributeError for me, on new post / new reply / edit post / delete post.

seems this should have been part of https://github.com/edx/edx-platform/commit/7d79f4fe3757a08e676bd906a392dd4bdf20961d but got left behind.  should this have been on our radar?

i grepped around and didn't see any other places we might need to look at, but a second pass from another pair of eyes couldn't hurt.