Tooling: add image compression

[chalin]

Originally posted by @mic-max in #869 (comment)

Making this part of the PR checks would be cool.
One way to implement it would be something like:

1. The PR either modifies or creates a PNG
2. Run a compression program on all the modified and created PNGs
3. If all the compressed images have the same file sizes in the PR, succeed. Otherwise fail.

Then you'd likely want to add some guidance that before uploading they run the same compression program locally to a README

[mic-max]

There is a GitHub app called ImgBot that does this that I recently discovered