Filter protocol-relative URLs in ExternalLinkFilter. Fixes issue #22742.

What does this MR do?

This MR contains changes so that protocol-relative links (e.g. //example.com rather than http://example.com or https://example.com) will be filtered by ExternalLinkFilter and appended with "rel" attribute with values "nofollow noreferrer".

Are there points in the code the reviewer needs to double check?

The regular expression used to filter protocol-relative link and sufficient tests were written.

Why was this MR needed?

To fix #22742 (closed)

Screenshots (if relevant)

N/A

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

#22742 (closed)

Closes #22742 (closed)