WebSubmit: guests support improved in /direct URL

• When accessing a /submit/direct URL as guest user, check if access is granted. If not, ask the user to log in prior to redirecting to the submission interface (and avoid an 'access denied' message). (closes #1240)

Reviewed-by: Samuele Kaplun <samuele.kaplun@cern.ch>
Signed-off-by: Ludmila Marian <ludmila.marian@gmail.com>