Format the list of optional_params to be clearer
Register latest change fields with Panopticon

This changes the Panopticon Registerer to include the `public_timestamp`
and `latest_change_note` fields as optional fields when registering
an artefact with Panopticon.
Merge pull request #252 from alphagov/panopticon-register-timestamp-and-change-notes

Register latest change fields with Panopticon