Add subtitles to homepage video

[machow]

Via CfP leadership, it would be helpful to have subtitles on the homepage video:

I think Walt designed it so people could pull out the gist based on the videos text / partial hard captions, but people should be able to experience all the language in the video even w/o audio.

[tblacknc]

Pass this around if you would:
https://pr-70.codeforphilly.sandbox.k8s.phl.io/