Display milestone due date for ticket if due date not set

If a ticket doesn't have a due date but it does have a milestone, show
the milestone's due date. Added `Ticket.due` property that selects the
appropriate due date according to that logic. Note that this only
affects the *display* of due dates. It would be nice if we could also
filter by milestone due dates.
Add milestone due dates to due-on filter for tickets

For tickets that don't have an explicitly set due date but that are
attached to a milestone, include their milestone due dates in the Due
On filter list. If a ticket has both an explicitly set due date and a
milestone due date, only the explicit due date will be included.

This filter now also ignores time completely.