various improvements

• Images: only show “did you know shortpixel” notice on Autoptimize settings pages (no more littering all over the backend)
• Images: update lazysizes from upstream
• Images: misc. improvements such as fix for PHP “undefined index” notice, updated copy, …
• HTML: rename HTML minify class from minify_HTML to AO_minify_HTML to avoid conflicts with e.g. W3TC
• Critical CSS: misc. improvements such as detect is_front_page before any other conditional, fix for conditional rules without an actual condition, improved debug logging, …
• JS/ CSS: fix for AO not optimizing multisite child sites when CDN set