register: make registration modular (step 1)

This changes the registration process from a 14-step (!) wizard to a module registration. The basic registration is now a 4-step wizard, and every other part of the registration can be accessed directly.

Each sub-part of the registration is now managed via a linked list system with previous and next steps (which can be configured dynamically), but for now only the basic registration is really wired up in that way.