editor(course): choosing "edit" on a course page doesn't open that page in the Editor

[hejtful]

How it should work:

1. login
2. navigate to https://de.serlo.org/mathe/51979/51555/uebersicht-berechnungsmethoden
3. press "edit" button
4. opens "Übersicht - Berechnungsmethoden" page

Additionally, CoursePage can be removed from "Editor Test" section in Serlo ___content page.