(WIP) Ex-60 As an EdX finance employee, I can “invalidate” unpaid purchases.

1) @cdodge Here i am just sharing the change set related to instructor dashboard where a fincance admin can invalidate or re-validate the sales. Currently I am only updating the "is_valid" field in the "invoice" model that will be helpful in identification of valid and invalid registration codes + some UI changes. 

2) The second part of this story as mentioned below is in pending state as waiting from the UX organization for the message pattern and any mockups. 

---- [ First-order implementation: when a student with an invalidated enrollment tries to access the courseware, they cannot access the courseware, and are instead presented with the message: "Payment for your place in this course is overdue. Please contact the person responsible for payment." Student messaging will be determined by UX (https://openedx.atlassian.net/browse/UX-835) ] 

3) @cdodge with reference to your email, you were saying that  we should also set the is_active flag to False on the courseware registration table. Are you pointing to [student_courseenrollment] model ? if YES then i was observing that when we set that flag to false then it redirects the user to course about page.

I will wait for your suggestions in order to complete this story. @Lyla-Fischer 

Thanks.

I feel like code is more readable when short failure cases are handled first. You can also use the [`JsonResponse`](https://github.com/edx/edx-platform/blob/master/common/djangoapps/util/json_request.py#L29) class. I'd suggest:
```python
if obj_invoice.is_valid:
    return HttpResponseBadRequest(_("The sale associated with this invoice has already been invalidated."))
obj_invoice.is_valid = False
obj_invoice.save()
message = _('Invoice number {0} has been invalidated.').format(obj_invoice.id)
return JsonResponse({"message": message})
```