Notification server fixes

[metenn]

• socket.io version bump (1.3.7 -> 4.7.2)
• removes "request" dependency as it's long deprecated - now using the fetch API
• some work towards typing and partially rewriting the javascript code
• uses minified socket.io on the client

These changes are already on szkopul.

[DietPawel]

@metenn will this close #45 ?

[metenn]

@metenn will this close #45 ?

Yes. It uses the latest socket.io and I made sure to verify there aren't any breaking changes and updated the code where there were any.