add owner role support (#1340)

This PR adds support to Owner roles.

The owner role has a similar access level as the admin, but it has the power to delete the account.

Besides that, the role has the following constraints:

- The role can only be transferred. So, only a user with the owner role can transfer the owner role to a new user
- It can't be assigned to users being invited
- It can't be assigned to service users

management/server/group_test.go

@@ -57,7 +57,7 @@ func TestDefaultAccountManager_DeleteGroup(t *testing.T) {
 		{
 			"integration",
 			"grp-for-integration",
-			"only admins service user can delete integration group",
+			"only service users with admin power can delete integration group",
 		},
 	}