Clean up schema for table web_hooks
Per https://gitlab.com/gitlab-com/infrastructure/issues/1709 (private to GitLab employees):
- Missing FK: project_id -> projects
- Missing FK: service_id -> services (8386 entries violate this rule)
- Missing FK: group_id -> namespaces
- Duplicate data: type could be enum or 4-byte integer
- Duplicate url is often repeated, may be more efficient in another table