Add GitHub action to compress images

[plata]

Ensure that app miniatures are small to improve repo sync times.

[madoar]

Will this compress images only once or will this repeat with every push? Should we compress all images or only ones that do not fulfill certain criteria? For example a certain file size threshold

[plata]

Reading https://github.com/marketplace/actions/image-actions#compress-on-demand-or-on-schedule, my understanding is that it should not touch files which are already compressed enough. Not completely sure though. Maybe we also need compressOnly: true.

I don't see a problem compressing all.