fix: fix video rendering on some iOS devices by switching to mp4

[dschlabach]

What changed? Why?
We had an issue where some webm files were not loading on certain iOS devices (not all, and seemingly not dependent on iOS version).

After testing, I was able to figure out that our webm files were encoded codec_name=vp9; when I converted this to VP8, it was supported on iOS simulators and in other browsers. The issue with VP8 files that I generated was that they were far too big in size [reference].

My next approach was to use a mp4 file with a poster attribute, but this sometimes causes a flicker once the user tries to play the video and it transitions from the poster to the video.

Finally, I've settled on using mp4s, which are not much bigger than the original webm files, and setting the timestamp to 0.001 in lieu of using a poster attribute.

Notes to reviewers

How has it been tested?

[vercel]

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[cb-heimdall]

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