Send public timestamp and change note to rummager

https://www.pivotaltracker.com/story/show/82952014

This changes Panopticon to support the new `public_timestamp` and `latest_change_note` fields added in alphagov/govuk_content_models#251. 

This also changes the `RummageableArtefact` model to include these new fields when rendering an artefact to be sent to Rummager.
This should probably be `strip_nil_keys`