Can check verified-ness and expiration date
End-to-end refunding with tests
Refactored to use signals; full test coverage
Fixes from Daian's CR
Response to Dave & Diana's CR
More response to Dave's PR
Added new copy, small CR fixes

* Changed text for "Are you sure you want to refund"...
* Addressed a CR comment adding clarity to student view code
* Converted UTC to pytz.utc
Moved #unenroll-modal to different template
Clarified wording in a docstring
Moved can_refund logic to CourseEnrollment