adds sso_identifier to /admin/users

[Katzenkralle]

Includes sso_identifier in JSON return of /admin/users

[Timshel]

I had added it only to /users/overview since it's the only one neeeded for the admin table.

But it would probably make sense to return the same json object in all of /users/overview, /users, /users/by-mail/<mail> and /users/<user_id> ?

[Katzenkralle]

Sure, all paths now return the same object