Switch to data_group and data_type in get response

Avoid exposing the data_set id in the api, as it is a numeric field
rather than a uuid, and we use data_group and data_type for creating
modules, and never require the id.
Merge pull request #248 from alphagov/data-group-and-data-type-in-admin-dashboard-get

Switch to data_group and data_type in get response