Autocomplete should send cache control headers

http://issues.bestpractical.com/Ticket/Display.html?id=21042

lib/RT/Interface/Web.pm
- implement ruz's suggestion for a 'cache-forever'
- https://tickets.bestpractical.com/Ticket/Display.html?id=91289#txn-1527031

share/html/Helpers/Autocomplete/autohandler
- change argument to Time per ruz's suggestion