Various Mermaid improvements (#18776)

* Various Mermaid improvments

- Render into iframe for improved security
- Use built-in dark theme instead of color inversion
- Remove flexbox attributes, resulting in more consistent size rendering
- Update API usage and update to latest version

* restart ci

* misc tweaks

* remove unneccesary declaration

* make it work without allow-same-origin, add loading=lazy

* remove loading attribute, does not seem to work

* rename variable

* skip roundtrip to DOM for rendering

* don't guess chart height

* update comment to make it clear it's intentional

* tweak

* replace deprecated 'scrolling' property

* remove unused css file

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

web_src/less/themes/theme-arc-green.less

@@ -455,10 +455,6 @@ img[src$="/img/matrix.svg"] {
   filter: invert(80%);
 }
 
-.mermaid-chart {
-  filter: invert(84%) hue-rotate(180deg);
-}
-
 .is-loading::after {
   border-color: #4a4c58 #4a4c58 #d7d7da #d7d7da;
 }