Add loading spinners and mermaid error handling (#12358)

- Add loading spinners on editor and mermaid renderers
- Add error handling and inline error box for mermaid
- Fix Mermaid rendering by using the .init api

web_src/js/markdown/content.js

@@ -1,5 +1,5 @@
 import {renderMermaid} from './mermaid.js';
 
 export default async function renderMarkdownContent() {
-  await renderMermaid(document.querySelectorAll('.language-mermaid'));
+  await renderMermaid(document.querySelectorAll('code.language-mermaid'));
 }