A fairly large change that replaces Zaps in the codebase with "Zapping"
which is a tagged union consisting of a resolved Zap and a Pending Zap.
These are both counted as Zaps everywhere in Damus, except pending zaps
can be cancelled (most of the time).
This adds private zaps, which have messages and authors encrypted to
the target. Keys are deterministically generated so that both the
receiver and sender can decrypt.
Changelog-Added: Private Zaps
