Add separate dealer reg page.

This adds a new URL, which is simply "preregistration/dealer_registration" - makes the link a bit prettier to hand out.

Also fixes #595.
Eventually, I'd like prereg_badge_types to be a bit more configurable - perhaps just moved into configspec.ini - and the logic for checking if different badge types can register moved to preregistration.py or decorators.py. For now, I just left groups as-is.