[Chore] Integrate imageflow instead of using ImgBot

[jawnsy]

It would be nice to integrate imageflow to compress images, rather than using the ImgBot service, because commits merged into the repository are immutable, so the larger images will forever be a part of our repository history, even if ImgBot later comes around to optimize them.

We could use imageflow or a similar tool as part of our build process, and then detect unoptimized images in CI (similar to how we handle prettier-formatted files), so that we avoid checking in large images in the first place.