PLT-5225 Added separate post types for certain system messages (#5193)

* Added separate post types for system_join_leave and system_add_remove posts

* Added username prop to channel deleted posts

* Changed slack import to properly generate the new join/leave messages

* Added username prop to channel update posts

* Moved Post(AddTo/RemoveFrom)ChanneMessage into app package

store/sql_post_store.go

@@ -93,7 +93,8 @@ func (s SqlPostStore) Save(post *model.Post) StoreChannel {
 		} else {
 			time := post.UpdateAt
 
-			if post.Type != model.POST_JOIN_LEAVE && post.Type != model.POST_ADD_REMOVE {
+			if post.Type != model.POST_JOIN_LEAVE && post.Type != model.POST_JOIN_CHANNEL && post.Type != model.POST_LEAVE_CHANNEL &&
+				post.Type != model.POST_ADD_REMOVE && post.Type != model.POST_ADD_TO_CHANNEL && post.Type != model.POST_REMOVE_FROM_CHANNEL {
 				s.GetMaster().Exec("UPDATE Channels SET LastPostAt = :LastPostAt, TotalMsgCount = TotalMsgCount + 1 WHERE Id = :ChannelId", map[string]interface{}{"LastPostAt": time, "ChannelId": post.ChannelId})
 			} else {
 				// don't update TotalMsgCount for unimportant messages so that the channel isn't marked as unread