Fix incomplete registers

[incorbador]

Description

Currently, if a user cancels the signup he can not restart the signup because the user itself is already existing in our backend as the first register call has been made.

Testing

After canceling my register attempt I should be able to start a new one with the same email address.

Implementation idea

Not sure yet what the best solution for that problem is:

• deleting the user from the app is not working (we can probably not allow that to be done over the public API if the user is unauthenticated)
• we could alter the backend to allow "overwriting" a user if that user is in incomplete state (not trivial though, if implemented in this simple form it could allow stealing between users if the perfectly time this) => for this task we probably have to talk to someone familiar with the backend API and bounce some ideas.