It already had one, but it didn't have a public key and it was using the old
mess of the Vervis.ActivityStreams module, which I'll possibly remove soon.
It's hopefully more elegant now.
This patch includes some ugliness and commented out code. Sorry for that. I'll
clean it up soon.
Basically there's a TVar holding a Vector of at most 10 AP activities. You can
freely POST stuff to /inbox, and then GET /inbox and see what you posted, or an
error description saying why your activity was rejected.
The actor key will be used for all actors on the server. It's held in a `TVar`
so that it can always be safely updated and safely retrieved (technically there
is a single writer so IORef and MVar could work, but they require extra care
while TVar is by design suited for this sort of thing).
