Show a success button at the top of the Progress page

### Add SHOW_PROGRESS_SUCCESS_BUTTON feature

This will show a button at the top of `lms/templates/courseware/progress.html` if the lowest nonzero grade cutoff has been reached.

Introduce two settings:
* `PROGRESS_SUCCESS_BUTTON_URL` is the href of that button (the course id is appended)
* `PROGRESS_SUCCESS_BUTTON_TEXT` is the text, defaults to "*Download your certificate*"

The change is pretty minimal. However, feedback on settings naming/placement and testing would be welcome.

*This is a test task for recruiting purposes* - @antoviaque