Document progressive registration flow

[cpoulain]

Start a wiki page to document the intended registration flow.

For now the registration form will remain as is with e-mail, username and password as required field, and the requirement that username and e-mail must be unique in the system.

[irjudson]

@cpoulain @percyliang @beaucephus Please review this and fix or comment. I'm closing this for now.

[irjudson]

I'm reopening to have Levert and Christopher review this (once I figure out their ids)

@MedicalTest @csrampey please review this and make sure it makes sense. Edit it inline if not.

[Zyzzx]

The fields that are in there are username, password and email. The progressive part is not built as that is new functionality and views need to be created.