[FIXED JENKINS-20327] When saving a list view, submit POST data with size...

[FIXED JENKINS-20327] When saving a list view, submit POST data with size proportional to the # of jobs selected for inclusion, not the number of options.
This can be done easily by setting json="true"; otherwise the default is to pass =false for each unselected job.
Ironically, even with this fix we are sending 2× what is needed,
since currently ListView.submit does not even check the json parameter for job names:
it only pays attention to top-level request parameters that the browser defines for selected checkboxes.
Ideally we could ask buildFormTree to ignore a given form element entirely, so as to avoid the wasted data.