Messenger

Network chat for messaging.
Currently implemented:
- chat work offline (reading messages received earlier)
- both group and private chats are created
- display the list and the ability to select all customers in the group
- implemented the ability to create, exit, join, rename, delete group and private chats
- display of the online client (with whom correspondence is conducted)
- real-time message notification when the recipient is offline
- sending message history to the user offline - online