Websubmit: fixes and improvements

3 commits:

WebSubmit: more robust javascript check
* Before running a custom JavaScript function on an element, check if the element is not undefined.

WebSubmit: print white space instead of None
* Avoids printing "None" (by passing None to string formatting). Instead, print a single white space.

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.