MA-141 Support webm encoding for VAL-enabled videos

https://openedx.atlassian.net/browse/MA-141

@ormsbee Can you do a review of this PR as well?  @lou-wang  would like to get it into this week's release.
No, it depends on the order that we list the desired profiles on L187.  Right now, we give preference to the "desktop_mp4" version.

It seems the video_module code is limited to supporting only one download_video_link.  But it's not the server, but the browser, that determines whether it needs to fallback onto the webm version instead.