Check for presence of an index for foreign keys
In line with https://docs.gitlab.com/ee/development/migration_style_guide.html#adding-foreign-key-constraints, we want to make sure rubocop checks add_concurrent_foreign_key
calls and makes sure there's an index on it.
See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21423/diffs#note_97917645 for an example.