Add a new config option for OAuth2 authentication sources: allow users to change their username.
In the case where OAuth2 is more like a social OAuth2 login there's no need to not allow users to change their username. The information how the user is linked to the authentication source is stored in different fields.
Resolvesforgejo/forgejo#687
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8714
Reviewed-by: 0ko <0ko@noreply.codeberg.org>
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Gusted <postmaster@gusted.xyz>
Co-committed-by: Gusted <postmaster@gusted.xyz>
