Handle tricky problems to get subtitles working on intro video

First and foremost, the mime type has to be supported by the web
server. Either you have to add a `.vtt` mimetype, *or* just put it
as a `.txt` file. I chose the latter since it doesn't require a
custom Apache configuration.

Second, even if you make it a `.txt` file, using a CDN does not seem
to work. I suspect that maybe CDNs do really weird things with
mimetypes when they server them, or maybe the <track> element just
really doesn't like it when the URL is 301'd. 🤷

Anyway, with these two changes: using a `.txt` file, and hosting the
file locally, the subtitles now work properly!

Note, if you use the `default` attribute in the <track> tag, it'll
turn them on by default. Leaving it off does allow the user to turn
them on in my tests in both Chromium and Firefox.