* API endpoint for certificate generation, an authenticated post with course
id requests that grading be carried out and a cert generated for
request.user in that course, using the usual grading and certificate
machinery (ie, it does not imply whitelisting, though whitelists and
blacklists will be respected)
- Logs each request as it comes in
- Calls xq.add_cert() and consequently, does grading synchronously on
this app host and then queues request for certificate agent.
- example usage:
```
curl --data "student_id=9999&course_id=Stanford/2013/Some_Class" http://127.0.0.1:8000/request_certificate
```
* 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.
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