Research + implement multiple sidecar subtitle rendition selection

[chrisbenincasa]

HLS master playlists are never refreshed by clients after stream startup. As such, any subtitle rendition we want available as a "sidecar" (pass through) stream needs to be defined "upfront" in the multi-variant playlist. We'll need to research + design for the following:

1. Allowing users to choose, per-channel, subtitle renditions that want included in the playlist (if they exist)
2. During playback, if an item starts streaming that doesn't include a backing stream that matches a selected rendition, then a faux (empty) subtitle stream needs to be inserted and continuously produce segments.
3. Creating the master playlist with the renditions and exposing that to the client

re: 2 -- we will probably want to package a small placeholder VTT file (jsut the header, WEBVTT should theoretically work) that is used as a dummy input for items that have gaps in renditions.