Auto enroll students even if there are multiple course modes

[ECOM-16](https://openedx.atlassian.net/browse/ECOM-16): Auto-register students as soon as they press the register button.

Changes:
* Implement auto-registration (based on @flowerhack 's proof-of-concept branch).
* Add tests for enrollment/unenrollment (somehow these weren't covered!)
* Slight change to `ModuleStoreTestCase` configuration to avoid importing XML courses we won't be using in the tests.

Not yet implemented:
* Template updates (although it does pass in the auto-registration flag to the context of the "choose your track" template)

I'm going to test this more thoroughly tomorrow, but I think this is ready for code-review.
@flowerhack @dianakhuang could you please review the auto-registration changes?
@jzoldak Could you please review updates to the test infrastructure?