Improve the AP async HTTP delivery API and per-actor key support

New iteration of the ActivityPub delivery implementation and interface.
Advantages over previous interface:

* When sending a ByteString body, the sender is explicitly passed as a
  parameter instead of JSON-parsing it out of the ByteString
* Clear 3 operations provided: Send, Resend and Forward
* Support for per-actor keys
* Actor-type-specific functions (e.g. deliverRemoteDB_D) removed
* Only the most high-level API is exposed to Activity handler code, making
  handler code more concise and clear

Also added in this patch:

* Foundation for per-actor key support
* 1 key per actor allowed in DB
* Disabled C2S and S2S handlers now un-exported for clarity
* Audience and capability parsing automatically done for all C2S handlers
* Audience and activity composition automatically done for Vervis.Client
  builder functions

Caveats:

* Actor documents still don't link to their per-actor keys; that should be the
  last piece to complete per-actor key support
* No moderation and anti-spam tools yet
* Delivery API doesn't yet have good integration of persistence layer, e.g.
  activity is separately encoded into bytestring for DB and for HTTP; this will
  be improved in the next iteration
* Periodic delivery now done in 3 separate steps, running sequentially; it
  simplifies the code, but may be changed for efficiency/robustness in the next
  iterations
* Periodic delivery collects per-actor keys in a
  1-DB-transaction-for-each-delivery fashion, rather than grabbing them in the
  big Esqueleto query (or keeping the signed output in the DB; this isn't done
  currently to allow for smooth actor key renewal)
* No support yet in the API for delivery where the actor key has already been
  fetched, rather than doing a DB transaction to grab it; such support would be
  just an optimization, so it's low-priority, but will be added in later
  iterations

th/models

@@ -118,6 +118,12 @@ Actor
     UniqueActorOutbox    outbox
     UniqueActorFollowers followers
 
+SigKey
+    actor    ActorId
+    material ActorKey
+
+    UniqueSigKey actor
+
 Person
     username            Username
     login               Text
@@ -161,40 +167,11 @@ Forwarding
     activity    RemoteActivityId
     activityRaw ByteString
     signature   ByteString
+    forwarder   ActorId
     running     Bool
 
     UniqueForwarding recipient activity
 
-ForwarderPerson
-    task   ForwardingId
-    sender PersonId
-
-    UniqueForwarderPerson task
-
-ForwarderGroup
-    task   ForwardingId
-    sender GroupId
-
-    UniqueForwarderGroup task
-
-ForwarderRepo
-    task   ForwardingId
-    sender RepoId
-
-    UniqueForwarderRepo task
-
-ForwarderLoom
-    task   ForwardingId
-    sender LoomId
-
-    UniqueForwarderLoom task
-
-ForwarderDeck
-    task   ForwardingId
-    sender DeckId
-
-    UniqueForwarderDeck task
-
 -- ========================================================================= --
 -- ========================================================================= --
 

th/routes

@@ -150,6 +150,8 @@
 
 /reply/#MessageKeyHashid                                 ReplyR            POST
 
+/people/#PersonKeyHashid/stamps/#SigKeyKeyHashid         PersonStampR      GET
+
 ---- Group ------------------------------------------------------------------
 
 /groups/#GroupKeyHashid                                 GroupR           GET
@@ -158,6 +160,8 @@
 /groups/#GroupKeyHashid/outbox/#OutboxItemKeyHashid     GroupOutboxItemR GET
 /groups/#GroupKeyHashid/followers                       GroupFollowersR  GET
 
+/groups/#GroupKeyHashid/stamps/#SigKeyKeyHashid         GroupStampR      GET
+
 ---- Repo --------------------------------------------------------------------
 
 /repos/#RepoKeyHashid                             RepoR              GET
@@ -186,6 +190,8 @@
 
 /repos/#RepoKeyHashid/enable-loom/#LoomKeyHashid  RepoLinkR          POST
 
+/repos/#RepoKeyHashid/stamps/#SigKeyKeyHashid     RepoStampR         GET
+
 ---- Deck --------------------------------------------------------------------
 
 /decks/#DeckKeyHashid                             DeckR           GET
@@ -203,6 +209,8 @@
 /decks/#DeckKeyHashid/follow                      DeckFollowR     POST
 /decks/#DeckKeyHashid/unfollow                    DeckUnfollowR   POST
 
+/decks/#DeckKeyHashid/stamps/#SigKeyKeyHashid     DeckStampR      GET
+
 ---- Ticket ------------------------------------------------------------------
 
 /decks/#DeckKeyHashid/tickets/#TicketDeckKeyHashid            TicketR            GET
@@ -248,6 +256,8 @@
 /looms/#LoomKeyHashid/follow                      LoomFollowR     POST
 /looms/#LoomKeyHashid/unfollow                    LoomUnfollowR   POST
 
+/looms/#LoomKeyHashid/stamps/#SigKeyKeyHashid     LoomStampR      GET
+
 ---- Cloth -------------------------------------------------------------------
 
 /looms/#LoomKeyHashid/cloths/#TicketLoomKeyHashid            ClothR            GET