Add after-install videos
Created by: felixfbecker
This adds the instruction videos to the after_install page. They are configured to behave like GIFs (mute, looping, autoplaying, no controls).
The videos are uploaded to this GCP storage folder so we don't bloat the extension package (they would be downloaded again on every update, and we'd have to source control them in the repo which is bad too).
Closes #14577 too