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.

modules/auth/auth_form.go

@@ -23,6 +23,7 @@ type AuthenticationForm struct {
 	AttributeSurname  string
 	AttributeMail     string
 	Filter            string
+	AdminFilter       string
 	IsActived         bool
 	SMTPAuth          string `form:"smtp_auth"`
 	SMTPHost          string `form:"smtp_host"`