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/private/internal.go

@@ -14,6 +14,7 @@ import (
 	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/gitea/modules/web"
 	"code.gitea.io/gitea/routers/common"
+	"code.gitea.io/gitea/routers/web/misc"
 	"code.gitea.io/gitea/services/context"
 
 	"gitea.com/go-chi/binding"
@@ -59,6 +60,7 @@ func Routes() *web.Router {
 	// Since internal API will be sent only from Gitea sub commands and it's under control (checked by InternalToken), we can trust the headers.
 	r.Use(chi_middleware.RealIP)
 
+	r.Get("/dummy", misc.DummyOK)
 	r.Post("/ssh/authorized_keys", AuthorizedPublicKeyByContent)
 	r.Post("/ssh/{id}/update/{repoid}", UpdatePublicKeyInRepo)
 	r.Post("/ssh/log", bind(private.SSHLogOption{}), SSHLog)