Orders: Add a tracking table for course run purchases

[rhysyngsun]

We need to be able to have a simpler way of determining which runs a learner has purchased. Currently you have to go through User -> Order -> Line -> Product -> CourseRun, which makes querying inefficient and messy.

I've proposed a join table that would track information on what course runs have been paid for, something like this:

class PaidCourseRun(TimestampedModel):
    user = models.ForeignKey(
        User, on_delete=models.CASCADE, related_name="paid_course_runs"
    )
    course_run = models.ForeignKey(
        CourseRun, on_delete=models.CASCADE, related_name="paid_course_runs"
    )
    order = models.ForeignKey(
        Order, on_delete=models.CASCADE, related_name="paid_course_runs"
    )

This would simplify the joins down to User -> PaidCourseRun -> CourseRun.

We would only create these for orders in the FULFILLED or REVIEW state and keep the records in the event of a refund.

Tasks

• Add a model similar to the above psuedocode
• Where we currently mark an order as fulfilled or in review state, add code to create and/or update a record
• In a PR that is merged and deployed after the above logic change is deployed add a migration that backpopulates records for existing orders

[dseaton]

@rhysyngsun @rachellougee I have confirmed that this works for me. Thank you!