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>

options/locale/locale_en-US.json

@@ -84,6 +84,7 @@
   "save": "Save",
   "add": "Add",
   "add_all": "Add All",
+  "dismiss": "Dismiss",
   "remove": "Remove",
   "remove_all": "Remove All",
   "remove_label_str": "Remove item \"%s\"",
@@ -3278,6 +3279,13 @@
   "admin.config.cache_test_failed": "Failed to probe the cache: %v.",
   "admin.config.cache_test_slow": "Cache test successful, but response is slow: %s.",
   "admin.config.cache_test_succeeded": "Cache test successful, got a response in %s.",
+  "admin.config.common.start_time": "Start time",
+  "admin.config.common.end_time": "End time",
+  "admin.config.common.skip_time_check": "Leave time empty (clear the field) to skip time check",
+  "admin.config.instance_maintenance": "Instance Maintenance",
+  "admin.config.instance_maintenance_mode.admin_web_access_only": "Only allow admin to access the web UI",
+  "admin.config.instance_web_banner.enabled": "Show banner",
+  "admin.config.instance_web_banner.message_placeholder": "Banner message (supports markdown)",
   "admin.config.session_config": "Session Configuration",
   "admin.config.session_provider": "Session Provider",
   "admin.config.provider_config": "Provider Config",