HackerOne reported issue: rel options not being added to external links in ADoc and other markup files

We received a HackerOne report that external links contained in AsciiDoc files are assigned target=_blank but are not also receiving the rel: noopener noreferrer tags to prevent tabnabbing. It looks like this will also be true of RDoc, textile, and other markup files.

We should only have to edit the lib/gitlab/asciidoc.rb and lib/gitlab/other_markup.rb files to add ExternalLinkFilter to the pipeline that employs SanitizationFilter already.