MM-34549 shared channels; add users to channel that were already sync'd (#17361)

Fixes a bug and adds a feature for shared channels:
- The Bug: when creating new shared channels, users that had already been sync'd via another channel were not added to the new channel's member list, since the users were not sync'd again. This PR sync's users per channel.
- The Feature: support custom statuses

services/sharedchannel/sync_send.go

@@ -300,7 +300,7 @@ func (scs *Service) updateForRemote(task syncTask, rc *model.RemoteCluster) erro
 		return nil
 	}
 
-	syncMessages, err := scs.postsToSyncMessages(posts, rc, scr.NextSyncAt)
+	syncMessages, err := scs.postsToSyncMessages(posts, task.channelId, rc, scr.NextSyncAt)
 	if err != nil {
 		return err
 	}
@@ -365,7 +365,7 @@ func (scs *Service) updateForRemote(task syncTask, rc *model.RemoteCluster) erro
 		}
 
 		// update NextSyncAt for all the users that were synchronized
-		scs.updateSyncUsers(syncResp.UsersSyncd, rc, nextSince)
+		scs.updateSyncUsers(syncResp.UsersSyncd, task.channelId, rc, nextSince)
 	})
 
 	wg.Wait()
@@ -470,9 +470,9 @@ func (scs *Service) updateNextSyncForRemote(scrId string, rc *model.RemoteCluste
 	)
 }
 
-func (scs *Service) updateSyncUsers(userIds []string, rc *model.RemoteCluster, lastSyncAt int64) {
+func (scs *Service) updateSyncUsers(userIds []string, channelID string, rc *model.RemoteCluster, lastSyncAt int64) {
 	for _, uid := range userIds {
-		scu, err := scs.server.GetStore().SharedChannel().GetUser(uid, rc.RemoteId)
+		scu, err := scs.server.GetStore().SharedChannel().GetUser(uid, channelID, rc.RemoteId)
 		if err != nil {
 			scs.server.GetLogger().Log(mlog.LvlSharedChannelServiceError, "error getting user for lastSyncAt update",
 				mlog.String("remote", rc.DisplayName),
@@ -486,12 +486,14 @@ func (scs *Service) updateSyncUsers(userIds []string, rc *model.RemoteCluster, l
 			scs.server.GetLogger().Log(mlog.LvlSharedChannelServiceError, "error updating lastSyncAt for user",
 				mlog.String("remote", rc.DisplayName),
 				mlog.String("user_id", uid),
+				mlog.String("channel_id", channelID),
 				mlog.Err(err),
 			)
 		} else {
 			scs.server.GetLogger().Log(mlog.LvlSharedChannelServiceDebug, "updated lastSyncAt for user",
 				mlog.String("remote", rc.DisplayName),
 				mlog.String("user_id", scu.UserId),
+				mlog.String("channel_id", channelID),
 				mlog.Int64("last_update_at", lastSyncAt),
 			)
 		}