(signup) Prevent anonymous LDAP binding for verification

We needed to add checks for empty username/password combinations because our
LDAP server will bind successfully with an anonymous login.
(signup) Add verification error notifications
(signup) Fix problem with POST request Content-Type

AngularJS sends POST data as JSON (Content-Type: application/json) by default,
but our auth.php server code (and most PHP server code in general) doesn't
accept JSON POST requests. So we just need to update the request header with
the proper Content-Type (application/x-www-form-urlencoded).