Define _display_name method for Response that refer to Audit.

Follow-up to [PR 512](https://github.com/reciprocity/ggrc-core/pull/512) to make sure that `Response`s have a meaningful event log entry, just like the `Request`s do.  Same problem as with `Request`s, that `Response`s lack a built-in unique identifier so it defines the `_display_name()` by the primary key and `Audit` it falls under.