Get the NHM DCAT profile working again
Get the NHM DCAT profile working again

Actual fixes:

- passing the correct number parameters to the `_add_triples_from_dict`
  and related functions. Not sure how this was working before - perhaps
  we upgraded dcat and that's when this broke.
- handling dates correctly, which broke when the versioned search
  backend came in.
- pulling the mime field not the format field from the images

Everything else is a formatting/niceness change.

NaturalHistoryMuseum/ckanext-nhm,NaturalHistoryMuseum/ckanext-nhm,NaturalHistoryMuseum/ckanext-nhm