Redmine 6 support

This is a fixed release for Redmine 6 (thanks to @liaham).
There are also some minor changes, see the CHANGELOG.md.

Mathjax imported from the Drawio URL

Now the Mathjax library is imported from the Drawio site, simplifying configuration and management.
Also added a couple of minor fixes/improvements on the handling of diagrams in XML format.
For more details, see the CHANGELOG.md.

Fixed some bug

Fixed a bug with the drawio_dmsf macro, and another related to MathJax.
See the CHANGELOG.md for details.

Fixed passing arguments to the drawio url

Now it's possible to pass configuration arguments in the Draw.io service url configuration option of the plugin.
For example, if you have a local Draw.io installation at http://drawio.local, you can configure the url as:

http://drawio.local/draw?stealth=1

to disable loading diagrams from external urls.

Fixed commonmark support

Fixed wiki toolbar with the common mark syntax.
Many thants to @liaham.

Fixed error when when checkig email

Fixed a small error related to emails.

Fixed working on Redmine 4

Version 'v1.4.3' introduced an error that prevented the plugin from working on Redmine 4.
This version fixes it, as well as some fixes in the integration with the WYSIWYG plugin.
See the 'CHANGELOG' for details.

Fixed toolbar buttons in the WYSIWYG editor plugin

With this release the toolbar buttons for inserting/editing the macros are available also in the WYSIWYG editor plugin.
The buttons are also available in the native editor when using the Redmine 5 experimental CommonMark Markdown (GitHub flavor).
This release has also some fix in the German translation.
See the CHANGELOG.md for details.

Diagrams included in PDF exports

Now diagrams are included in the PDF export of pages from Redmine.
There are other small fixes, see the CHANGELOG.md for the details.

Potential XSS security vulnerability

Fixed a potential XSS security vulnerability on macro dialogs.
The vulnerability is really minimal as the form is never submitted, but can made happy automatic scanning tools.

Other German translations have also been added in this version.