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 #213 from edx/dave/fix_inflated_cert_grades

Initialize MakoMiddleware manually during certificate grading runs.