Rudimentary time tracking

These changes add a "Time worked" input to the Post Reply and Post Internal Note forms, so staff members can indicate how many minutes they have spent on activities associated with that reply or note.  When nonzero, this information appears in two places:  First, the timeworked associated with each thread item is displayed on the ticket view next to the thread item's author's name.  Second, the sum of timeworked for each ticket is shown on the list of tickets in the staff panel, next to the threadcount.

There is one new database field, (prefix)ticket_thread.timeworked .  I had to alter the table manually to add this field to my running install, but I've updated install-mysql.sql in an attempt to have the field created for a new deployment.