Minimal refactoring for rolling certificates

* 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
Adding 'Grade Me' YAML file
Certificates: Additional flags for regenerate_user

* -G allows explicit setting of grade condition (e.g., a string such as
  'Distinction', 'Pass', 'F', 'B') for the certificate, regardless of
  grader output for the user.
* -T allows explicit request of a particular certificate template to be
  applied to a given user's certificate regeneration, i.e.:
    -T doctor_distinction.pdf
  to generate a certificate of distinction for doctors.
WIP - passing title to cert agent