Support unfolding and refolding of sensitive toots.

Support unfolding and refolding of sensitive toots.

There's a new highlight type for this, so that it can apply to
InReplyTo lines as well as statuses themselves.

The set of unfolded sensitive statuses needs to be shared between lots
of views of the data, so I've put it in the TuiLogicalState, under a
RefCell so that ActivityStates can all borrow it easily, not just to
look up in but also to modify.