add YAML-tag `external` to courses (course/index.md)
add YAML-tag `external` to courses (course/index.md)

When the `external` tag is set, a click on the course will redirect to the
specified url.

In example, one can add an external arduino course by creating `arduino/index.md`
with the contents:
```
---
title: Arduino
external: https://kodegenet.no/track/arduino
---
```

arve0/codeclub_lesson_builder,arve0/codeclub_lesson_builder,arve0/codeclub_lesson_builder