and add the ability to record the fulfillment time

* Make currency a property of the Order (for validation purposes)
* Remove null=True from Char fields
* Use InheritanceManager for subclassing OrderItem
* Change VerifiedCertificate to better handle some new use cases
* Cleaned out old migrations
* Tests!

when in a dev environment.

Move course modes to common and enable in Studio admin

Make it so that students who are not enrolled cannot see the forum