Keep track of paid course runs

[annagav]

We should not delete PaidCourseRun on unenrolling the student. This model was created for tracking purposes and was meant to keep track of the payment status of the enrollment.

It was first introduced and described in this issue #738

The deletion of the model was introduced in this PR #1474.

[pdpinch]

@annagav is there a reason why this came up?

[annagav]

Here is another case of a missing PaidCourseRun https://github.com/mitodl/hq/issues/3929.

When we defer a user the PaidCourseRun gets deleted.
I think we need to add states to paidCourseRun instead of deleting.

Paid course run is used in multiple places:

1. ecommerce/models.py has_user_purchased_same_courserun- to determine if the user has already paid for the run.
2. in _validate_enrollment_post_request to validate enrollment request (to make sure it is not a duplicate request.)