New communication tool

Talkwall shows itself not very useful by various reasons, so we migrate to GitHub for discussions:

https://github.uio.no/IN3020-4020/Discussions

All students (registered for the course) should be able to login with the university account (if you have a problem, drop me an email to egork@ifi.uio.no). To ask a question, go to Issues tab and rise an issue. You may close it if you are happy with an answer.