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/base/head_banner.tmpl

@@ -0,0 +1,11 @@
+{{$banner := ctx.CurrentWebBanner}}
+{{if $banner}}
+<div class="ui info message web-banner-container">
+	<div class="render-content markup web-banner-content">
+		{{ctx.RenderUtils.MarkdownToHtml $banner.ContentMessage}}
+	</div>
+	<button type="button" class="btn dismiss-banner link-action" aria-label="{{ctx.Locale.Tr "dismiss"}}" data-url="{{AppSubUrl}}/-/web-banner/dismiss">
+		{{svg "octicon-x"}}
+	</button>
+</div>
+{{end}}

templates/base/head_navbar.tmpl

@@ -176,3 +176,4 @@
 		</div>
 	{{end}}
 </nav>
+{{template "base/head_banner"}}