Introduce \OCA\Mail\Service\Html

:construction: 
We should also add `target="_blank"` in addition to `rel="noreferrer"` to every link to allow modern browsers (i.e. Chrome) to open this page in another process to further harden security.

This can be done via http://htmlpurifier.org/live/configdoc/plain.html#HTML.TargetBlank

See http://blog.chromium.org/2009/12/links-that-open-in-new-processes.html for details about this fancy security feature.