Replace assign-on-reopen with claim-on-response

If a ticket is currently unassigned and your respond to it, it's yours. If a
ticket is closed and later reopened due to a new message, the last
respondent will always be assigned if available at the time.
Merge pull request #238 from greezybacon/feature/claim-on-response

Replace assign-on-reopen with claim-on-response

Reviewed-By: Peter Rotich <peter@osticket.com>