Allow recordings to use mp4 file format

[jonas2515]

The recording server can be configured to output h264 encoded mp4 files, those are way more compatible across platforms compared to webm or mkv.

It would be nice if the allowlist could be extended to allow recordings in mp4 format. Is there specific reasons against it, otherwise I'm happy to create a PR for it!

[nickvergessen]

that was specifically chosen, as back in the days it was the only codec that played in all supported browsers and mobile clients.
Not sure this is still the case, but it's kind of a pre-requirement.

[bentuna]

WEBM is not compatible with macOS yet. Quicktime (and thus also the Finder) cannot open the files or generate a thumbnail. Safari cannot open the files - although it can play WEBM videos when they are embedded in a website.