Add automated checks for src/_img/avatars/*

[mathiasbynens]

All images in src/_img/avatars/* follow certain conventions:

• JPEG format
• the @2x version is named [email protected] and must be exactly 192×192px
• the @1x version is named first-last.jpg and must be exactly 96×96px
• all images must be optimized i.e. below a certain file size threshold (10 kB?)
  • I tend to use Squoosh with MozJPEG @ quality=75, and then run the result through ImageOptim.app with “optimization level” set to “insane” for further savings.

All of the above could use automated CI checks, so that we don’t accidentally merge PRs that deviate from the convention.