Better support of PDF export by storing images of diagrams at diagram save

Currently we are using SVG for the PDF export of diagram. However this creates two issues:

1/ It does not currently work for private document as the SVG is handled by PDF including an image tag to a URL call to the wiki to provide the SVG

2/ PDF export of SVG does not support html inside the SVG which the diagram app uses extensively.

Instead to improve PDF export I suggest storing the image as an attachment on save in the same way as storing the SVG currently by using the drawio javascript export to image code. Then the PDF and print view of the document would insert the image instead of the live diagram.

The output would be much nicer as it would be sized to the width of the PDF and also not have the glitches of the batik rendering of SVG