Filter protocol-relative URLs in ExternalLinkFilter
Description
The ExternalLinkFilter adds the "rel" attribute with values "nofollow noreferrer" to links to external sites, such as in
- this link to example.org and
- this link to example.org
It does not filter protocol-relative links though, such as
Proposal
The filter should also add the "rel" attribute with "nofollow noreferrer" to those kind of URLs.