Web.ActivityPub: Add createTarget field

I'll use this for C2S to allow client to state who the tracker actor is. It's
still possible to do without it, by HTTP GETing the ticket's context and
checking whether we got an actor, or a non-actor with ticketsTrackedBy. Tbh I'm
adding createTarget simply because it's easier for coding, no need for a custom
variant of actor fetching :P

src/Vervis/API.hs

@@ -362,6 +362,7 @@ createNoteC host (Note mluNote luAttrib aud muParent muContext mpublished source
                         , noteSource    = source
                         , noteContent   = content
                         }
+                    , createTarget = Nothing
                     }
                 }
             tempUri = topLocalURI

src/Vervis/Federation.hs

@@ -266,7 +266,7 @@ handleSharerInbox now shrRecip (ActivityAuthRemote author) body =
     case activitySpecific $ actbActivity body of
         AcceptActivity accept ->
             sharerAcceptF shrRecip now author body accept
-        CreateActivity (Create obj) ->
+        CreateActivity (Create obj _target) ->
             case obj of
                 CreateNote note ->
                     sharerCreateNoteF now shrRecip author body note
@@ -296,7 +296,7 @@ handleProjectInbox now shrRecip prjRecip auth body = do
             ActivityAuthLocal local -> throwE $ errorLocalForwarded local
             ActivityAuthRemote ra -> return ra
     case activitySpecific $ actbActivity body of
-        CreateActivity (Create obj) ->
+        CreateActivity (Create obj _target) ->
             case obj of
                 CreateNote note ->
                     projectCreateNoteF now shrRecip prjRecip remoteAuthor body note

src/Vervis/Migration.hs

@@ -477,6 +477,7 @@ changes hLocal ctx =
                                             , noteSource    = msg
                                             , noteContent   = contentHtml
                                             }
+                                        , createTarget = Nothing
                                         }
                                     }
                                 tempUri = topLocalURI

src/Web/ActivityPub.hs

@@ -1086,20 +1086,22 @@ instance ActivityPub CreateObject where
 
 data Create u = Create
     { createObject :: CreateObject u
+    , createTarget :: Maybe (ObjURI u)
     }
 
 parseCreate :: UriMode u => Object -> Authority u -> LocalURI -> Parser (Create u)
 parseCreate o a luActor = do
     obj <- withAuthorityT a $ parseObject =<< o .: "object"
     unless (luActor == attrib obj) $ fail "Create actor != object attrib"
-    return $ Create obj
+    Create obj <$> o .:? "target"
     where
     attrib (CreateNote note)     = noteAttrib note
     attrib (CreateTicket ticket) = ticketAttributedTo ticket
 
 encodeCreate :: UriMode u => Authority u -> LocalURI -> Create u -> Series
-encodeCreate authority actor (Create obj) =
-    "object" `pair` pairs (toSeries authority obj)
+encodeCreate authority actor (Create obj target)
+    =  "object" `pair` pairs (toSeries authority obj)
+    <> "target" .=?    target
 
 data Follow u = Follow
     { followObject  :: ObjURI u