Update Vulnerability Feedback factory to include creation of issuables

Per discussion in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9326#note_143084413, we should update the Vulnerabilities::Feedback factories to reflect real usage:

1. When creating a feedback record of type issue, it should have a related issue
2. When creating a feedback record of type merge_request, it should have a related merge_request (Handled with https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9326)