Delay rules now hold every kind of request: scripts, styles, images, fonts and media, plus files from other sites (CDNs), to test skeletons, spinners and slow third-party files. In Firefox this uses a blocking webRequest
listener, so fetch/XHR delays are exact while a page loads.

Fixes:
• Settings > Back no longer leads to a 404.
• Rules imported from a link or a JSON file are always rebuilt from their conditions, so a file can no longer hide a different redirect or header rule (security).
• The first fetch/XHR requests of a page no longer wait about a second while the extension's background wakes up.
• The "XHR" request type filter now also matches fetch() in Delay, Modify API Response and Modify Request Body rules.
• Delay rules with a page domain filter now delay scripts, styles, images and fonts.
• Fonts and other CORS requests are no longer delayed twice.