Flowerhack/feature/refunds for certificates

When a user unenrolls in a course, if (1) they purchased a verified certificate and (2) they are unenrolling within two weeks of the course start date, this sends an email to billing so that the student may receive a refund.

There's a few quality errors that I wasn't sure whether to fix or suppress—i.e. wasn't sure how to do the tests in test_access without calling a protected method—so thoughts on that would be lovely

@ormsbee @dianakhuang 

LMS-1210