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
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/42335.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/233862