Embed VTT into the service file

[DonRichards]

Instead of relying on a VTT file and a MP4 file separately, we should be combining them within the media service file.

Here's a blog on how this is accomplished within FFMPEG for multiple language embedding.

https://ytmt-stag.com/post/00002_ffmpeg/

Lots more benefits than I've mentioned. This came up in a tech call and it was suggested to add a ticket here for it.

[seth-shaw-unlv]

I did a bit of looking and couldn't confirm that the major browsers support using the embedded caption/subtitle tracks. We should test creating a combined file and then viewing it in browsers before we put a lot of effort into automating a solution.

[adam-vessey]

Not... really sure this is a good idea for general use, as some may want to adjust their VTT by hand... having to extract and re-embed in order to manipulate it seems rather round-about? Otherwise, having to go rederive a service file just to change the VTT seems expensive, without a good chunk of logic avoiding re-encoding if it's just the VTT being changed... but would/should still result in a new version of the video file, wasting storage... but yeah... gets rather complex.