Improve protocol over websocket + write docs for API usage

[codeofmochi]

• Define more precise message types (avoid "any" if possible)
• Define clear protocol, easy to decode
• Write docs for protocol usage
• Modify back-end and front-end implementation with new types

[codeofmochi]

Doc will be available under API.md

See https://github.com/graasp/graasp-websockets/blob/master/API.md