Add created and modified fields and migration

These have been added to make admin of data-sets easier in future.
Add created and modified fields to fixtures
Add created and modified fields to list display
Merge pull request #149 from alphagov/add-created-modified-fields

Add created and modified fields and migration
Add the published field to dataset list view

Finishes #72683710
Merge pull request #150 from alphagov/add-published-field-dataset-list

Add the published field to dataset list view
Add '(UTC)' to created and modified labels