Users page: username instead of login name as col header to avoid confus...

...ion with other backends

It causes confusion like in #3592 and may be applied to other backends. "Username" is more general and thus hopefully a better designator.

What do you think @jancborchardt @karlitschek @icewind1991 (and others) ?


(LDAP explanation: In LDAP, there is not THE login name. It is what you specify in settings, it can be many – like username and email – and configuration may be changed by Admin. Therefore LDAP backend has a fixed username that is used internally.)