Add honor code to registration dialog.

* Add the "honor code and terms of service" link to the registration dialog.
* This does NOT yet support separate terms of service / honor code checkboxes.  We'll need to add that before merging back to master, but this should be enough to support the edx.org use-case.

@AlasdairSwan @stephensanchez Please review.
@AlasdairSwan This is the new field you'll get from the /registration API end-point.