Switch to converged handlers than handle both local and remote activities

I was writing a topicLocalInvite handler when I realized how cumbersome
it's becoming, to have separate handlers for local activities. While it
allows me to pick custom specific message names and parameters (which is
why I took that approach in the first place), it causes a lot of
duplication and complexity (because I have to write the remote-activity
handlers anyway; adding local ones doesn't reduce complexity).

So this commit switches the entire system to communicate only using
AP/FF activities, including between local actors.

th/models

@@ -228,11 +228,14 @@ FollowRemote
     UniqueFollowRemoteFollow follow
     UniqueFollowRemoteAccept accept
 
---FollowRequest
---    person PersonId
---    target FollowerSetId
---
---    UniqueFollowRequest person target
+FollowRequest
+    actor  ActorId
+    target FollowerSetId
+    public Bool
+    follow OutboxItemId
+
+    UniqueFollowRequest actor target
+    UniqueFollowRequestFollow follow
 
 Follow
     actor  ActorId