Generate 'Better' Thumbnails

[Arthi-chaud]

Note: This is a long-term issue. I don't expect to solve this with a single PR

Generating thumbnails is easy: we use a keyframe somewhere around the middle of the video. However, the thumbnails do not always look good: sometimes, keyframes are blurry or nothing of interest is in frame.

We could match videos with an external provider, and download their thumbnails. But two challenges arise: we need to have an correct matching algorithm (we do not want an alternative version of a video to be identified as the main version), and the probability that such provider exists is low (YouTube would be good, but not sure we could find everything we need, especially since we can have 'rare' videos that are not on there + we wouldn't want to have like a 'Vevo' logo on the thumbnails).

Another approach would be to generate 'better' thumbnails. For example, we could select all keyframes that have a face in them, and pick one of them