feat: optional kroki/plantuml svg caching (#2047)

* feat: Caching kroki svgs #2020

server/modules/rendering/markdown-plantuml/renderer.js

@@ -116,7 +116,7 @@ module.exports = {
 
         token = state.push('uml_diagram', 'img', 0)
         // alt is constructed from children. No point in populating it here.
-        token.attrs = [ [ 'src', `${server}/${imageFormat}/${zippedCode}` ], [ 'alt', '' ], ['class', 'uml-diagram'] ]
+        token.attrs = [ [ 'src', `${server}/${imageFormat}/${zippedCode}` ], [ 'alt', '' ], ['class', 'uml-diagram prefetch-candidate'] ]
         token.block = true
         token.children = altToken
         token.info = params