Implements GET of /users/X/inbox and POST to /users/X/outbox for authenticated clients to consume. It's not a full implementation of the client to server interactions but I'm trying to make a subset.
For reference see https://www.w3.org/TR/activitypub/#client-to-server-interactions
Ref #444 (closed)
I'm new to elixir and new to pleroma so I realise there will be a lot to fix in this PR :)