Materials of archived courses should be freezed/should not change

Right now courses, for example runs/2017/pyladies-praha-jaro/info.yml, just reference lessons by their path, for example beginners/testing.

What happens when the lesson moves to other directory or even disappears? It would break content of some previous course if someone doesn't dedicate their time to update all such references.

What happens when the lesson changes? I think the content of the old course should be preserved as is, as it was presented and teached.

Since all materials are currently in git on Github, accessible through git and Github APIs, this could be doable for example by creating a new branch after each course ends and material for the archived course could be rendered from the given branch instead of master.

Beginne mit runs/2017/pyladies-praha-jaro/info.yml und verfolge, wie die Pfade der Lektionen gerendert werden. Untersuche anschließend den in der Issue erwähnten auf Git und der GitHub API basierenden Ansatz; abgeschlossen ist die Aufgabe, wenn archivierte Kursmaterialien weiterhin verfügbar und unverändert bleiben, sobald aktuelle Lektionen verschoben, entfernt oder bearbeitet werden.