WIP: screenshots.py: Create single-file output

This might fix the screenshot issue:

• Currently only works on Firefox (and maybe Safari) because Chrome doesn't let users fetch/XMLHttpRequest the current file (when a file:// URL).
  • Works in Chrome if using python3 -m http.server to host.
• Now only generates one screenshots output file, screenshots.html.
• CSS-based & JS-based diff (both)
• Code is still really messy & needs to be cleaned up