search: Properly pagenate search results

Also re-implement the "exclude answered tickets" and "exclude assigned
tickets" from open queue options.
Add est_duedate column to ticket

This will contain the effective due date of the ticket — either the due date
or the estimated overdue time of the ticket based on the SLA settings. It
will be maintained with ticket updates.

orm: Add greater support for expressions in order_by
orm: Convert Team to ORM
Add `lastupdate` field to %ticket

And manage it with updates to tickets. This will ease the sorting and
display of tickets based on the time of last update in the ticket queues
Don't construct ticket objects to view ticket queue

This significantly reduces the time necessary to render the ticket queue
page with the ORM.
oops: Fix saving of searches
Fixup stats and display of assigned tickets