Prefer last message date to reopened date for default sort order

- Tickets will recent replies from the user floats to the top.
- Priority still tops the effective
Use GREATEST to get the most recent activity date
Change recent activity date as opposed to create date on ticket view
Merge pull request #161 from protich/feature/sortorder

Ticket view sort order.

Reviewed-By: Jared Hancock <jared@osticket.com>