Support Other Channels/Languages

## Summary

There has been a request to include a separate youtube channel with videos hardsubbed in Spanish. The general implications of this include:

1) We need to support pulling videos from a whitelist of youtube channels.
2) Videos and segments should be labeled with a language.
3) We should provide more general search filters where users can choose the youtube channel, language, unprocessed videos, etc.

## Plan (WIP)

This needs to be broken down into stages.

- [ ] Add whitelist of channel ids so videos can at least be available to timestamp.
...TODO: Detail remaining steps

## Context

From Slack:

```
Ricardo Luis Gutiérrez - Argentina 12:34 PM
Hi Justin,
I had a conversation with Greg about whether it could be feasible to include the videos from our little Spanish-subtitled collection (38 clips of Tom's videos, hardsubbed) into the pool of videos-to-be-timestamped. These videos would thus be rendered useful for either English or Spanish speakers. Greg redirected me to you as you are developing the tool.
We cannot do "closed captions" because 1)they are hardsubbed, 2)we don't have the subtitle files anymore, as I DID lose them all.
I envision regular time-stampers doing this as a normal part of their job, the only special thing is that we would be translating into Spanish the snippets they generate. The Spanish team were consulted to see if they agreed and yes, we are available to translate the snippets.
This looks simple enough to me, no need for added complexity. Perhaps we could use some notification when videos of this batch are timestamped, just to notice and proceed with the translation.
By the way, Marco Camacho and I are co-facilitators for the Spanish translation team. So both of us look forward to your opinion.
Just in case you don't have the link:

```
[MY BIG TOE en español](https://www.youtube.com/channel/UCECSQRaC-BvYTlGyyI0rByA)

No files, tests, or entry points are identified. Start by breaking the proposal into staged requirements for channel whitelisting, video and segment language labels, and search filters; define the interfaces and acceptance criteria for each stage before locating implementation areas.

Written by the indexing model from the issue text.