Initialize MakoMiddleware manually during certificate grading runs.

Without this, problems fail to load because they can't find how to
render themselves, and the certificate generation grading run will
get an inaccurately low count of the possible points a user could
get (anything they didn't see will be omitted), inflating their
grade during certificate calculation and making it inconsistent
with their Progress page.
Merge pull request #206 from edx/rc/2013-06-18

Rc/2013 06 18
Merge remote-tracking branch 'origin/release' into rc/2013-06-26