Add warning when a site is not served via HTTPS

[westonruter]

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.

[westonruter]

Another key reason why this is needed is images just start throwing validation errors for no apparent reason. The reason is the HTTP protocol is not allowed, at least for the noscript > img inside of the amp-img.

[westonruter]

Note that Site Health in core now has this:

[westonruter]

Add a warning on the AMP settings screen when not on HTTPS, similar to the object caching warning.

In addition, both HTTPS and object caching should be added as Site Health entries.