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.

runs/2017/pyladies-praha-jaro/info.yml から始めて、レッスンのパスがどのようにレンダリングされるかを追跡します。次に、issue で言及されている Git と GitHub API ベースのアプローチを調査します。現在のレッスンが移動、消失、または編集された場合でも、アーカイブされたコース資料が引き続き利用可能で変更されない状態になれば完了です。