feat: improved user creation validation

[jmorel]

Signed-off-by: Jérémy Morel jeremy.morel@owkin.com

Description

• Adds validation for username so that the actual creation doesn't fail with an IntegrityError
• Adds validation on empty password before using the configured validators. This is necessary because the zxcvbn validator will raise a list index out of range exception on an empty password string. See IndexError: list index out of range, on empty password dwolfhub/zxcvbn-python#64
• Decodes the url encoded kwargs.get('username') so that the username is properly handled (John Doe is a valid username).

How has this been tested?

Manually.

Checklist

• changelog was updated with notable changes
• documentation was updated

I added to this PR decoding of the "username" part of the URL when editing users.

[sergebouchut2]

It would be really nice to add some unit tests and prevent from future regression.

[cboillet-dev]

looking good to me but it would be worse adding a few validation tests in backend/users/tests/test_user.py

[jmorel]

By popular demand (wink wink @cboillet-dev @sergebouchut2), let there be tests!