DB: Migration: Fix error in mig 447, accidentally creation CollabTopicLocal

Repo/Loom/Deck created one in their migrations, but CollabTopicAccept isn't
itself a topic, it's just a helper, so it needs to *find* the relevant
CollabTopicLocal rather than create a new one.

src/Vervis/Migration.hs

@@ -2539,8 +2539,11 @@ changes hLocal ctx =
         (Just $ \ (Entity topicTemp (CollabTopicLocal447 collabTemp)) -> do
             collabs <- selectList [] []
             for_ collabs $ \ (Entity topicID topic) -> do
+                maybeLocal <- getKeyBy $ UniqueCollabTopicLocal447 $ collabTopicLocalAccept447Collab topic
                 localID <-
-                    insert $ CollabTopicLocal447 $ collabTopicLocalAccept447Collab topic
+                    case maybeLocal of
+                        Nothing -> error "No CollabTopicLocal for this Collab"
+                        Just l -> return l
                 update topicID [CollabTopicLocalAccept447CollabNew =. localID]
 
             delete topicTemp