(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.

Wait a minute, let me take another step back here. This function is used to verify that the user is putting a number into this field. There's a better way of doing that: use [`<input type=number>`](http://www.html5tutorial.info/html5-number.php). It's a HTML5 extension that tells the browser "this input only accepts numbers". Not all browsers support it, but most do these days. And you should be validating this on the server as well, so that it doesn't matter if a browser doesn't support it -- the error message on the server will inform the user of what's going on.