Move record_show action to versioned_datastore

[jrdh]

Not sure why I hadn't noticed this before, but this action shouldn't be in this extension, it should be in the https://github.com/NaturalHistoryMuseum/ckanext-versioned-datastore. I noticed this because I was using the record_show action in the ckanext-iiif extension and realised using this action would require a dependency from ckanext-iiif to ckanext-nhm which makes no sense when all you're trying to do is get hold of a record.

Not sure how many of our extensions moving this will affect but if we keep the name and signature the same it shouldn't be a massive problem (🤞🏼)