en: Refactored duplicate code in online_chat

[parvezmrobin]

The changes made and their justifications are as follows

Change	Justification
meesage → message	Typo
# ... → pass	Synchronizing .ipynb and .py
def receive_friend_request(self, friend_id): → def check_pending_friend_requests(self):	In real-world scenario, we do not immediately react to a friend request. Rather, when we have time we check the list of pending friend requests and accept or reject them.
Moving add_user and remove_user functionality to Chat	Both PrivateChat and GroupChat share these functionality. Thus, it is only convenient to move them to the parent class.