Fix unallowed characters in username

[azmeuk]

Nextcloud does not allow accentuated or special character in the usernames:
nextcloud/server#21313

However sometimes nextcloud-oidc-login is plugged to a IDP where users already have accentuated characters.

I suggest that nextcloud-oidc-login automatically fixes unallowed characters in usernames: replacing accents with unaccented characters and removing other unallowed characters.

[pulsejet]

Sounds good. This needs to be configurable since it potentially opens up attack vectors. If the IdP has a user azmeuk, then I can create another user àzmeuk in the IdP and pretend to be azmeuk in Nextcloud.

A possible solution would be to suffix the username with some kind of hash of the original, whenever any transformation is done.