Avoid "mixed content" message when loading images

Problem/Motivation

I get the following error:

Mixed Content: The page at '' was loaded over HTTPS, but requested an insecure element ''. This request was automatically upgraded to HTTPS, For more information see

Steps to reproduce

Run Drupal behind a reverse proxy on port 80. In my case, this is Traefik, a tool used in production environments.

Proposed resolution

Avoid the protocol in image URLs, for example by using //example.com/image.jpg instead of http://example.com/image.jpg.

Remaining tasks

While the attached patch solves the issue for images without a style, it remains for images loaded via a style.

User interface changes

None.

API changes

None.

Data model changes

None.