Proof of concept: mermaid

[as-op]

Testing mermaid integration:

• Include ckeditor/mermaid plugin
• mount diagrams in work package descriptions and activity comments
• error in console when changing into edit mode with ckeditor
  (probably a timing bug in ckeditor/mermaid)
• Github adds view controls to their MermaidJS, making very large diagrams usable which would be displayed far too small otherwise
  

Showstopper:

• No export to PDF: Mermaid heavily utilize browser layouting. For converting diagrams into valid SVG or PNG https://github.com/mermaid-js/mermaid-cli uses a headless browser via puppeteer, which installs several hundreds of MB of Chromium or Firefox support files. Server Side Support mermaid-js/mermaid#3650

Screenshots: