Sign up form: UX improvements

[g123k]

What

• After reading this article, I think we should improve the sign-up process to better highlight when a field is OK or have an incorrect value.
• Linked to feat: Sign up form: try to highlight the issue #2535

Part of

• 👥 User management (tracker) #1382

[teolemon]

After a discussion during the team meetup yesterday, I had a feeling that it would be worth mockuping what a wizard signup would look like, as opposed to a form.
All of the data is asked abrupty, and I was wondering whether a step by step illustrated wizard could be any nicer.
It would also allow in the future to introduce backup features and the likes.
Name, Email, Username, Password, Producer status, Newsletter consent, Legal consent.