Many AMP components require HTTPS to function. When a page is not served as HTTPS, an AMP component can just fail to work (e.g. forms) leaving the user in confusion. See Automattic/jetpack#11595. While we originally thought in #969 to just handle the warning in the PWA plugin, since it is so important to serve as HTTPS we should show a warning on the AMP settings screen similar to how we show a warning when not using an object caching plugin.

This would be unnecessary if core starts to show a non-HTTPS warning as part of Health Check before we get around to it.