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.
I think that's the right approach.  There's currently some crazy special case handling if Stanford theming is enabled, but I think adding a new `tos` field to that setting will work.

I'm planning on doing that quickly this morning, so I don't think we need a separate story.  Just wanted to unblock the JavaScript work.