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

templates/repo/editor/edit.tmpl

@@ -41,9 +41,7 @@
 						data-markdown-file-exts="{{.MarkdownFileExts}}"
 						data-line-wrap-extensions="{{.LineWrapExtensions}}">
 {{.FileContent}}</textarea>
-					<div class="editor-loading">
-						{{.i18n.Tr "loading"}}
-					</div>
+					<div class="editor-loading is-loading"></div>
 				</div>
 				<div class="ui bottom attached tab segment markdown" data-tab="preview">
 					{{.i18n.Tr "loading"}}