Use https image-proxying service for the “Image report” feature

[sideshowbarker]

When users access a validator instance running under a non-insecure (https) origin (e.g, https://html5.validator.nu/) and use the “Image report” feature with an insecure (non-TLS http) document, the results page will cause the user to see a mixed-content warning indicator in the browser chrome.

To avoid causing users to get mixed content, we could have the checker use an https image-proxying service for all the images displayed in the “Image report” output; example:

https://sideshowbarker.net/tests/proxied-image.html

Details at https://carlo.zottmann.org/2013/04/14/google-image-resizer/

[sideshowbarker]

Closed in favor of #76