Every link with a target now gets rel="noopener noreferrer", and only
_blank is kept as a target. A named target such as target="foo" left
the page it opened a handle on this one — browsers imply noopener for
_blank alone — which it could use to send the tab to a lookalike login.
The share page turned links into the private workspace into plain text
with a regex over the sanitised HTML. A pattern over markup can end
inside an attribute, and on an engine that does not escape < and > in
attribute values a crafted link title became markup. The rewrite now
happens in renderMarkdown, on the parsed page.