Until now, Artemis assumed the default branch in git is master.
Since many different systems are in the process of renaming the default branch from master to main, Artemis should also get independent and rather read the default branch from the git configuration.

Needed changes: replace all places in which master is "hard coded" and try to retrieve the default branch from the git configuratio

This was the result of a discussion we had way back, where we decided to not let users break out of the "Artemis space" since we can't really guarantee what will happen as only the usage with the Artemis website is tested.
Let's also say a teaching assistant links a code example and the student clicks on it. He is now outside of our domain and has no obvious way of going back, apart from re-open