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

---------

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

templates/admin/layout_head.tmpl

@@ -1,5 +1,5 @@
 {{template "base/head" .ctxData}}
-<div role="main" aria-label="{{.ctxData.Title}}" class="page-content {{.pageClass}}">
+<div role="main" aria-label="{{.ctxData.Title}}" class="page-content {{.pageClass}}" {{if .dataGlobalInit}}data-global-init="{{.dataGlobalInit}}"{{end}}>
 	<div class="ui container fluid padded flex-container">
 		{{template "admin/navbar" .ctxData}}
 		<div class="flex-container-main">