Existing benefactors can create registration references

One-sentence description

Existing benefactors can create registration references

Current behaviour / Reproducing the bug

1. Create a registration or renewal for a benefactor
2. Login as benefactor
3. Create reference for the new benefactor
4. Works

Expected behaviour

1. Create a registration or renewal for a benefactor
2. Login as benefactor
3. Create reference for the new benefactor
4. Error