Use URL param for registration flow

So, soon, the registration flow will use URL params and redirections
instead of session variables. Here's how:

  GET /registrations/new
    -> /people/:id/edit if needs to input personal details
    -> /people/:id/waiver if hasn't signed waiver

Each of those steps will either (by default) redirect to the GET version
of the same thing (i.e. #new) or if ?next=registration param is passed
it, it will redirect back to /registrations/new, so the registration
process can again redirect the user accordingly.