Support staff locking on unit page

STUD-1873
@cahrens I believe that unpublished changes with no release date have no state. The states are additive as follows (I think):
 is-published - true if the item has been published
 is-scheduled - true if the item has a release date assigned
 is-released - true if the item has a release date in the past
 is-staff-only - true if the item has been marked staff only
 has-changes - true if the item has changes since being published (not so sure about this one)

I don't believe there is an is-draft in the latest revision, which means I should remove the draftBit variable if that's correct.

@frrrances @talbs Can you confirm this? We probably need this list written down on the wiki somewhere so we are all working from the same definitions.

While I'm here, what does 'bit' signify? I find it a strange word to parse in the code and the HTML.