This will help search engines know these links point to external sites
and it'll make it possible to style these links using the
`[rel~=external]` selector. AFAIK, assistive techonologies don't use
this attribute to notify people about external links, though.
This is a funny one, because we were accidentally opening them in the
same window without intending to do so since commit 928312e21, since the
`sanitize` method removes the `target` attribute. So the test we're
adding already passed without these changes.
