Force amp-img > noscript > img fallbacks to have HTTPS src/srcset

[westonruter]

If an image is in post content with an http protocol, this will persist into the noscript > img fallback when it is converted to amp-img (see #1832). While an amp-img allows http protocol, the noscript > img does not. See fix to AMP spec here ampproject/amphtml#21686. To prevent a lot of validation errors from appearing for sites that have HTTP images, we should consider automatically converting the noscript > img elements to HTTPS.