Refactored; sessions are assigned to models

- websocket is now application-wise
- connection with server is established upon initializing a client
instance
- User is bound to the client instance after login and unbound upon
logging out
- each application pages can reach the websocket client instance and use
it for retrieving data from the server and sending data to the server
- reworked socket messages' structure to include operation and data in
the message
- added handling of the following messages:
From server to client: "updateSessions", "updateModel"
From client to server: "getSessions", "getModel", "publishModel"
- added buttons to open and finish sessions (functionality is under
construction)