WebSubmit: improved guests support 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>