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.