Redmine 6 support

Mathjax imported from the Drawio URL.

Fixed some bug

Fixed passing arguments to the drwaio url

Fixed common mark support

Fixed error when sending mails

Fixed integration with the WYSIWYG plugin

Fixed also working with Redmine < 5.

Fixed toolbar buttons in the WYSIWYG editor plugin

Diagrams included in PDF exports

* now diagrams are included in the PDF export of pages.
* other small fixes

Fixed possible XSS