Consider visibility of jobs and job chains in the HTML interface

The attribute visible="no" that is returned by Job Scheduler <answer> elements for jobs and job chains should be considered by the HTML interface that hides such jobs and job chains respectively.

Additionally a JavaScript variable _force_visibility=true in custom.js could be used to enforce the previous behaviour that makes jobs and job chains visible (and their history accessible) should they have been executed before, i.e. this switch makes the HTML interface ignore the attribute visible= as returned by <answer> elements.