Restore RoleImplication _display_name method for log.

Follow-up to [PR 522](https://github.com/reciprocity/ggrc-core/pull/522) to correct display issue with event log: `Context` objects are now shown on the event log, but their corresponding `RoleImplication` changes lack a display name.  This had been created before and commented out.  I'm restoring it so that it properly functions for debugging until we can settle on a better way to represent this.