Handle "is writting" state

[Benvii]

Think about implementing an "is writting" state when users are writing in a channel :

• required having websocket in place : [Back] Add flask-socketio to back and front #252
• use the activity-bar props : Fixes #107 added activity bar #111 (comment)
• write a chat specification, that will contain an event about "is writting"

More to specify.