fix: encode videos in yuv420p to support Firefox

Firefox does not support yuv444p format:
https://bugzilla.mozilla.org/show_bug.cgi?id=1368063

The suggested solution from the bug-tracker is to
reencode the video using ffmpeg -pix_fmt yuv420p option.

The following command was used:

ffmpeg -i old_video.mp4 -pix_fmt yuv420p out.mp4

Fixes #1685.

Author: danielleontiev