The authentication settings forms have always just been a flat list of
fields, without any real organization and, in some cases, no sane order.
That made some settings forms, like for LDAP, a bit confusing.

This change adds support for fieldsets for auth forms, and makes use of
them for Active Directory, Standard Auth, and LDAP.

Visually and functionally tested each form. Counted the fields to make

sure that nothing went missing.