Alerting: Split grafana and lotex routes (#44742)

* split Lotex and Grafana routes
* update template to use authorize function for every route

pkg/services/ngalert/api/authorization.go

@@ -0,0 +1,11 @@
+package api
+
+import (
+	"github.com/grafana/grafana/pkg/middleware"
+	"github.com/grafana/grafana/pkg/web"
+)
+
+func (api *API) authorize(method, path string) web.Handler {
+	// TODO Add fine-grained authorization for every route
+	return middleware.ReqSignedIn
+}