VTT utilities added to convert SRT files to VTT and shift time codes

I'm not sure how you'll want to deal with uploading/storing of subtitle files, but we frequently have to convert SRT files to VTT files, and occasionally we have to shift the timing of the cues by a quarter of a minute or so.

I'm assuming we'll need to do that going forward, and it would be nice to have it automated. Let me know if this needs some cleaning up, or if there's something better we could be doing.