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/less/index.less

@@ -1,5 +1,7 @@
 @import "~font-awesome/css/font-awesome.css";
 @import "./vendor/gitGraph.css";
+@import "./features/animations.less";
+@import "./markdown/mermaid.less";
 
 @import "_svg";
 @import "_tribute";