[#1560] Favor thread flags while ordering threads.
Sort threads first by flags desc, then by date desc. Flag values also
appear to be sorted in decsending order, so Sticky sorts before Announcement.
That gives following exact semantics for flags:
Sticky - sort at the top of the thread list.
Announcement - show this thread in the header of each forum page.
So, if you want an Announcement to sort at the top of thread list,
make it also Sticky. Seems nice and logical.
Signed-off-by: Paul Sokolovsky <email@example.com>