Also, display a message on their dashboard.

LMS-1475

* Adds regenerate_user command, which lets you run certificates for one
  particular user in one particular class, regardless of whether they
  have a pre-existing certificate.

Move mitxmako initialization to a startup module, called by lms.startup and cms.startup for both management commands and when run via wsgi

[DEVPAIN-7]

There was a potential though unlikely race condition because the .save()
was after the certificate request was put on the queue.

Now calling .save() immediately after we update the
cert.

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.

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.

Add, remove or update and existing certificate

Adding, removing and updating certs