External Drives Module got dropped, we should revive it

https://twitter.com/UID_/status/1005795008581169152 

Excellent reminder that a little lesson on why you really shouldn't plug random drives into your computer is definitely worth including. I [started a draft](https://github.com/OpenNewsLabs/field-guide-security-training-newsroom/blob/master/drafts/external%20drives.md), but it's just a few notes. 

We should migrate that into a full lesson. 