Add MP4 conversion utility to convert a meeting's media to MP4 for use with Zoom sharing

[mtdvlpr]

Just recently, we had to meeting in Zoom only, because of a storm that made it unsafe to go outside. I had to use the old version with the MP4 conversion feature to be able to easily present the media through Zoom from a laptop at home without a second screen.

We can combine this feature with the export folder feature to make the export folder contain the converted MP4 files.

[sircharlo]

Working on this in #3756