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>

routers/web/auth/auth.go

@@ -162,6 +162,11 @@ func consumeAuthRedirectLink(ctx *context.Context) string {
 }
 
 func redirectAfterAuth(ctx *context.Context) {
+	if setting.Config().Instance.MaintenanceMode.Value(ctx).IsActive() {
+		// in maintenance mode, redirect to admin dashboard, it is the only accessible page
+		ctx.Redirect(setting.AppSubURL + "/-/admin")
+		return
+	}
 	ctx.RedirectToCurrentSite(consumeAuthRedirectLink(ctx))
 }