Instance-wide (global) info banner and maintenance mode (#36571)

The banner allows site operators to communicate important announcements
(e.g., maintenance windows, policy updates, service notices) directly
within the UI.

The maintenance mode only allows admin to access the web UI.

* Fix #2345
* Fix #9618

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Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

web_src/js/features/repo-editor.ts

@@ -197,5 +197,5 @@ export function initRepoEditor() {
 
 export function renderPreviewPanelContent(previewPanel: Element, htmlContent: string) {
   // the content is from the server, so it is safe to use innerHTML
-  previewPanel.innerHTML = html`<div class="render-content markup">${htmlRaw(htmlContent)}</div>`;
+  previewPanel.innerHTML = html`<div class="render-content render-preview markup">${htmlRaw(htmlContent)}</div>`;
 }