In ActorFetchShare, use insertBy' to allow non-shared insertions

Before this patch, the shared fetch used plain insert, because it relied on
being the only place in the codebase where new RemoteActors get inserted. I was
hoping for that to be the case, but while I tweak things and handle fetching
URIs that can be an actor or a public key (for which ActorFetchShare isn't
sufficient without some smart modification), I'd like concurrent insertions to
be safe, without getting in the way of ActorFetchShare.

With this patch, it now uses insertBy', which doesn't mind concurrent
insertions.

src/Vervis/RemoteActorStore.hs

@@ -451,8 +451,7 @@ addVerifKey h uinb vkd =
 
 actorFetchShareSettings
     :: ( YesodPersist site
-       , PersistUniqueRead (YesodPersistBackend site)
-       , PersistStoreWrite (YesodPersistBackend site)
+       , PersistUniqueWrite (YesodPersistBackend site)
        , BaseBackend (YesodPersistBackend site) ~ SqlBackend
        , HasHttpManager site
        )
@@ -468,7 +467,8 @@ actorFetchShareSettings = ResultShareSettings
                 manager <- getsYesod getHttpManager
                 eactor <- fetchAPID' manager actorId h lu
                 for eactor $ \ actor -> runDB $
-                    insertEntity $ RemoteActor lu iid (actorInbox actor) Nothing
+                    let ra = RemoteActor lu iid (actorInbox actor) Nothing
+                    in  either id (flip Entity ra) <$> insertBy' ra
     }
 
 fetchRemoteActor