#4755 Combining consecutive user join/leave system messages to single message and few other changes. (#5945)

fix 7 and 8

remove @ at "{username} joined the channel"

refactor and update test

store/store.go

@@ -170,6 +170,7 @@ type PostStore interface {
 	Overwrite(post *model.Post) StoreChannel
 	GetPostsByIds(postIds []string) StoreChannel
 	GetPostsBatchForIndexing(startTime int64, limit int) StoreChannel
+	GetLastPostForChannel(channelId string) StoreChannel
 }
 
 type UserStore interface {