Add form field flag for field to suppress the field if the value is blank or none, and the field is not editable

[timbl]

When a field in a form is output-only, uneditable, and the current value of the predicate is the empty string, or there are times when the field is not useful and should be suppressed.

See SolidOS/contacts-pane#13
where in the discussion we see that showing an empty Note field or Role field is not useful, and so the field should just not be displayed.