Feature request: Allow username selection

[BrutalBirdie]

It happened to use that the OAuth2 / OpenID provider did not provide a username.
Thus, we used the fallback to mail.
This resulted in people getting the same username over and over again.
Why?
Well, they have their own mail servers and mail inboxes and use e.g.: admin@foo.bar, admin@bar.foo, admin@mydomain.local resulting in automatically assigning the usernames admin 0, admin 1 and admin 2.

So a third fallback option to allow users to choose the username would be great.