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]

common/djangoapps/mitxmako/shortcuts.py

@@ -17,6 +17,7 @@ from django.http import HttpResponse
 import logging
 
 import mitxmako
+import mitxmako.middleware
 from django.conf import settings
 from django.core.urlresolvers import reverse
 log = logging.getLogger(__name__)