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 (closed)