Alternatively allow e-Mail Adresses for login instead of username

[kainhofer]

If a user has an account set up (i.e. a login name set), it should be possible to use the e-Mail address instead of the user name for login. Other systems like Nextcloud allow this kind of login, so our users are used to giving their e-Mail instead of the username.

[Fasse]

The email adress should be only the email adress of the default email field EMAIL and not of other configured email fields. There should also be an option to disable that feature.

The option could be "Allow login with email adress of profile field Email" and could be checked per default.