Allow to download recorded video in MP4 format

[darkdarkdragon]

Right now /recordings endpoint constructs HLS manifest with .ts segments in it. We also want to download recording in MP4 format.
We will add two URL query params: format and rendition. If format=mp4 then we will take rendition (source by default), will read all the .ts segments one-by-one, transmux them to mp4 on the fly and return that mp4.