Implement favouriting and boosting posts.

Implement favouriting and boosting posts.

For these uses of selection, we have to support different keystrokes
to fave and unfave, which means we also need to know which keystroke
is valid when. So I've had to plumb the Client through to all of the
methods that move the selection, because those methods need to
retrieve the newly pointed-to status and cache whether it's already
faved/boosted. Otherwise draw(), which can't mutate things, can't
find that out to display the right status line.