SAML: change the config option for making SAML UI accessible to org Admins (#67399)

* change from role grant overrides to SAML UI specific config option

* update permissions needed to access SAML UI

* PR feedback: change config name, change required perms to write, add a comment

pkg/services/accesscontrol/models.go

@@ -16,9 +16,8 @@ var ErrInternal = errutil.NewBase(errutil.StatusInternal, "accesscontrol.interna
 // RoleRegistration stores a role and its assignments to built-in roles
 // (Viewer, Editor, Admin, Grafana Admin)
 type RoleRegistration struct {
-	Role                RoleDTO
-	Grants              []string
-	AllowGrantsOverride bool
+	Role   RoleDTO
+	Grants []string
 }
 
 // Role is the model for Role in RBAC.