playlists: Verify all videos in a playlist use a specific thumbnail template

[ekimekim]

We should be able to associate a thumbnail template with a playlist such that:

• [ ] All videos that match that playlist should use that thumbnail template by default
• [ ] A warning can be raised (eg. in the sheet as an output column) if any videos don't use the template
  • This would probably require some way to manage exceptions, eg. if one video gets a fully custom thumb instead.

The goal here is to prevent forgetting to use a custom template when available for a linked series of videos.