Ignore URLs starting with // in Markdown links

## What does this MR do?

Render `[foo](//bar/baz)` as `<a href="//bar/baz">foo</a>`.

## Why was this MR needed?

`[foo](//bar/baz)` currently renders as `<a href="//bar/gitlab-org/gitlab-ce/master/baz">foo</a>`

## What are the relevant issue numbers?

fixes #7032

See merge request !5677