Media transcoding service

[Zasa-san]

split from #6326

Right now we serve audios and videos ‘as is’. The file the user uploaded is the one that will be served by us for playback on the client. This has a number of problems as described in the original issue.

We need a service that can transcode media files into friendly formats, and consider a quality that’s adequate for the places where we display them in the client.

We still would keep the original file so that users can download it, and the transcoded version would be used for playback.

[RafaPolit]

After the streamer is implemented, this could be an interesting addition: #6141

[RafaPolit]

To be developed with #6520