[v3] recording thumbnails

[hiroshisuga]

Is your feature request related to a problem? Please describe.
On GLv2 thumbnails are displayed for the list of recordings, although the feature is hidden anyhow (3 thumbnails from the default presentation slides shown). Some tweaking enables showing them nicely (e.g. #2639). It would be great if it revives on GLv3.

Describe the solution you'd like
Showing thumbnails on the recording list. Thumbnails should be generated from the slides. Not necessarily as much as 3 thumbnails, just one is probably enough. Preferably the room owner can select one from the list of miniature slides.

Describe alternatives you've considered

Additional context
The thumbnails have helped my students to remember the lecture easily. A possible problem is that the page loading could become slow, especially for loading the thumbnail candidates when the presentation has many slides.