WebSubmit: no double-submit

* Introduces JQuery trick to avoid double submitting a form
  in WebSubmit.
  (closes #1020)

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>
Merge branch 'maint-1.1'

* maint-1.1:
  WebSubmit: no double-submit