MDL-32584 PHP: Add check for expired password

Switched to using plugin setting for forcechangeexpiredpassword within auth_ldap
Added checks for new flag called auth_passwordexpired
Auth_passwordexpired is unset when password has not expired
Login page redirects to changepassword when auth_passwordexpired or auth_forcepasswordchange are set
Added language strings for auth_ldap's forcechangeexpiredpassword setting