HTML: ignore __duplicate__ formatting markers but /*preserve*/ stuff like this
Add support for Mailman's non-standard signature delimiter

There's a perfectly valid standard for how a signature separator shall look
like, but nope, Mailman just *has* to use something very special.

Let's just follow notmuch's example here and match even this garbage. On the
other hand, let's play it a bit safer and put a minimal length limit so that we
don't match "innocent" stuff like "__" etc.