Fix error 500s creating projects concurrently

If multiple projects within a group are created concurrently, a validation error due to duplicate ProjectAuthorization entries could be triggered. This can happen due to a race condition: if the user's or group's authorizations get refreshed before the Projects::CreateService does this, we can get a database unique key violation.

To fix this, we use safe_find_or_create_by! to ensure that a duplicate entry doesn't get inserted.

This fixes a regression introduced in GitLab 13.4 via !42335 (merged).

Closes #233862 (closed)