Automatically compress PNG images

Created as part of #3384 (closed)

pngbot will only run when a merge request is open, and will compress the PNGs added in the last commit and commit the compressed images back to the branch.

PNGs are first converted form 24bit to 8bit (256 colors) using pngquant before being losslessly compressed using zopflipng. This is a conservative compression strategy that we can adjust in future.

pngbot will report it's actions on the merge request via a comment which will be updated as more images are processed by pngbot.

See source here: https://gitlab.com/jramsay/pngbot