Set IsAdmin using LDAP

The IsAdmin flag is set based on whether the admin filter
returned any result. The admin filter is applied with the user dn
as the search root.

In the future, we should update IsAdmin as well on each login.
Alternately, we can have a periodic sync operation.

conf/locale/locale_en-US.ini

@@ -761,6 +761,7 @@ auths.attribute_name = First name attribute
 auths.attribute_surname = Surname attribute
 auths.attribute_mail = E-mail attribute
 auths.filter = User Filter
+auths.admin_filter = Admin Filter
 auths.ms_ad_sa = Ms Ad SA
 auths.smtp_auth = SMTP Authorization Type
 auths.smtphost = SMTP Host