Add support for discontinued course runs

[rhysyngsun]

We need to be able to discontinue a course run during a run. This would involve setting enrollment_end_date and end_date to the past, but a lot of our logic is based around course runs always receiving frozen grades. Because of this a past run without frozen grades is considered a current run. We need an additional state between that is for a discontinued run so that learners can enroll in subsequent runs without us having to generate fake grades.

This should be a boolean field, defaulted to False, and dashboard.api.get_status_for_courserun updated to handle this before the check for has_frozen_grades. Theremay be other place where we need to handle this so some additional due diligence is probably warranted.