* Studio advanced setting added, "certificates_show_before_end", which
  determines whether a course should permit certificates to be
  downloadable by students before the coures's end date has passed.
* Modifications to dashboard view and templates to allow display of
  certificate download links before course has ended. (XXX: may declare
  failing students as failing before the course has ended.)
* To test, turn the setting on in a course which hasn't ended yet, and
  force certificate generation for a student, then check their
  dashboard.
* TODO:
  - needs tests
  - needs pep8/pylint