Add mitxmako.middleware to fix workers

If mitxmako.middleware has ever been imported, then render_to_string
works correctly (which is why we didn't see this in the rest of
edx-platform). However, if it hasn't (like in a celery worker), then
using mitxmako.middleware errors out.

[LMS-1085]
Merge pull request #923 from cpennington/fix-mitxmako-middleware-import

Add mitxmako.middleware to fix workers