Alerting: Fix anonymous access to alerting (#49203)

* introduce a fallback handler that checks that role is Viewer.
* update UI nav links to allow alerting tabs for anonymous user
* update rule api to check for Viewer role instead of SignedIn when RBAC is disabled

pkg/services/accesscontrol/accesscontrol.go

@@ -129,6 +129,11 @@ var ReqGrafanaAdmin = func(c *models.ReqContext) bool {
 	return c.IsGrafanaAdmin
 }
 
+// ReqViewer returns true if the current user has models.ROLE_VIEWER. Note: this can be anonymous user as well
+var ReqViewer = func(c *models.ReqContext) bool {
+	return c.OrgRole.Includes(models.ROLE_VIEWER)
+}
+
 var ReqOrgAdmin = func(c *models.ReqContext) bool {
 	return c.OrgRole == models.ROLE_ADMIN
 }