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.
Could the settings.REGISTRATION_EXTRA_FIELDS work for this? Or would you want honor to optionally not show up at all?