WebSubmit: better filtering of reserved filenames

• When a value is submitted for a field named with a reserved keyword, still allow the submission of this value is the form was indeed on the page that submitted it (either page user is coming from, or declared to be coming from, as in /submit/direct cases). (closes #1107)

• Extends WebSubmit admin guide with documentation about reserved element names.

• Adds a warning on WebSubmit admin interface upon creation of element with reserved name.