LP#1709521 Webstaff show recent patrons

LP#1709521 Webstaff show recent patrons

Adds a new library setting 'ui.staff.max_recent_patrons' which
specifices the number of recently accessed patrons staff may quickly
refetch from the browser client interface.  If no value is set, it
defaults to 1 for backwards compatibility.

If the value is 0 or less, no recent patrons may be retrieved and the
'Retrieve Last Patron' action is hidden.  If the value is > 1 a new
'Retrieve Recent Patrons' menu item appears.  When clicked, the list
of recent patrons is displayed in the patron search interface, most
recently accessed patrons sorted to the top.

For privacy, the recent patrons list expires from the browser when the
authenticion token expires.

Signed-off-by: Bill Erickson <berickxx@gmail.com>