WebSubmit: support for elements' custom_level

* Adds support for optionally setting a custom_level ('M' for mandatory or
  'O' for optional) for each element through the element description,
  overriding the default level. Useful for dynamically produced elements.

  Signed-off-by: Nikolaos Kasioumis <nikolaos.kasioumis@cern.ch>
WebSubmit: print white space instead of None

* Avoids printing "None" (by passing None to string formatting).
  Instead, print a single white space.

  Signed-off-by: Nikolaos Kasioumis <nikolaos.kasioumis@cern.ch>
WebSubmit: more robust javascript check

* Before running a custom JavaScript function on an element, check if
  the element is not undefined.

  Signed-off-by: Nikolaos Kasioumis <nikolaos.kasioumis@cern.ch>
Merge pull request #10 from kasioumis/master-websubmit-fixes-and-improvements

Websubmit: fixes and improvements