MM-3543/MM-35436: fix thread being marked unread when updating reply post (#17645)

Summary
Threads should not be returned as unread if a reply post has been edited or reacted to since the time the thread was last viewed

Ticket Link
https://mattermost.atlassian.net/browse/MM-35436
https://mattermost.atlassian.net/browse/MM-35437

store/sqlstore/thread_store.go

@@ -121,7 +121,7 @@ func (s *SqlThreadStore) GetThreadsForUser(userId, teamId string, opts model.Get
 		model.Post
 	}
 
-	unreadRepliesQuery := "SELECT COUNT(Posts.Id) From Posts Where Posts.RootId=ThreadMemberships.PostId AND Posts.UpdateAt >= ThreadMemberships.LastViewed"
+	unreadRepliesQuery := "SELECT COUNT(Posts.Id) From Posts Where Posts.RootId=ThreadMemberships.PostId AND Posts.CreateAt >= ThreadMemberships.LastViewed"
 	fetchConditions := sq.And{
 		sq.Or{sq.Eq{"Channels.TeamId": teamId}, sq.Eq{"Channels.TeamId": ""}},
 		sq.Eq{"ThreadMemberships.UserId": userId},
@@ -147,7 +147,7 @@ func (s *SqlThreadStore) GetThreadsForUser(userId, teamId string, opts model.Get
 			LeftJoin("ThreadMemberships ON Posts.RootId = ThreadMemberships.PostId").
 			LeftJoin("Channels ON Posts.ChannelId = Channels.Id").
 			Where(fetchConditions).
-			Where("Posts.UpdateAt >= ThreadMemberships.LastViewed").ToSql()
+			Where("Posts.CreateAt >= ThreadMemberships.LastViewed").ToSql()
 
 		totalUnreadThreads, err := s.GetMaster().SelectInt(repliesQuery, repliesQueryArgs...)
 		totalUnreadThreadsChan <- store.StoreResult{Data: totalUnreadThreads, NErr: errors.Wrapf(err, "failed to get count unread on threads for user id=%s", userId)}
@@ -334,7 +334,7 @@ func (s *SqlThreadStore) GetThreadForUser(userId, teamId, threadId string, exten
 		model.Post
 	}
 
-	unreadRepliesQuery := "SELECT COUNT(Posts.Id) From Posts Where Posts.RootId=ThreadMemberships.PostId AND Posts.UpdateAt >= ThreadMemberships.LastViewed AND Posts.DeleteAt=0"
+	unreadRepliesQuery := "SELECT COUNT(Posts.Id) From Posts Where Posts.RootId=ThreadMemberships.PostId AND Posts.CreateAt >= ThreadMemberships.LastViewed AND Posts.DeleteAt=0"
 	fetchConditions := sq.And{
 		sq.Or{sq.Eq{"Channels.TeamId": teamId}, sq.Eq{"Channels.TeamId": ""}},
 		sq.Eq{"ThreadMemberships.UserId": userId},