Make sure YouTube API key is unset by default.

Default for the EDXAPP_YOUTUBE_API_KEY variable in edx/configuration repository is "PUT_YOUR_API_KEY_HERE" rather than empty/null.

Set it to null in default vars to prevent failing YouTube XHR requests when loading course units containing videos in the Studio or LMS.

A valid YouTube API key can still be set via extra settings.

Testing Instructions

1. Spawn a new VM.
2. SSH into the new VM.
3. Make sure YOUTUBE_API_KEY is set to null in /edx/app/edxapp/lms.auth.json and /edx/app/edxapp/cms.auth.json.
4. In the Studio, open a unit that contains at least one YouTube video.
5. Make sure no error is show.
6. Check the browser console to make sure there are no failed YouTube API XHR requests.

Reviewers:

• @itsjeyd