Logistration: enrollment cleanup and a few fixes

Changes:

* Move redirection/enrollment to AccessView and trigger it using an event.
* Ensure window.isExternal is loaded
* Don't set a default for terms of service (this was causing registration to fail with the default sandbox config)
* For paid courses, add the course to the cart and redirect to the shopping cart view.

Reviewers: @rlucioni @AlasdairSwan 
FYI: @dianakhuang 
I agree that this is pretty poorly named, but there's a pretty substantial risk to moving it, since pages in `edx-platform` may be searching for it using the old path.