RFC: Improve account setup form

# Problem

Based on verification_fields_needed, and by just adding stand-alone logic, it should be possible to implement client side validation for at least parts of the account setup form as well as add other improvements.

At the very least, we ought to be able to:

* Make all fields required, on all steps of the process.
* Render a "verification in process" status for various steps of the process.
* Optionally: If stripe reports problems with some fields, somehow highlight that on the client.