LP1850555 Item location selector improvements

LP1850555 Item location selector improvements

* Ensure that the desired location is always available in the selector
  regardless of whether the staff have permission to use the location.

* Display org unit short name for selected locations simimilar to how
  they are displayed in the selector dropdown.

* Adds an <Unset> option in cases where the new 'required' flag is set
  to false.  This explicitly makes it possible for staff to clear the
  value.

* Gracefully handle cases where locations from no org units are eligible
  for display.

Signed-off-by: Bill Erickson <berickxx@gmail.com>
Signed-off-by: Jane Sandberg <sandbej@linnbenton.edu>