Add the ability to return non-nested responses

This functionality means we no longer need to use JavaScript on the client
side to flatten the data.

The code has been written so it should be relatively straightforward to strip
out the grouped/nested code in the future when Backdrop changes to only return
flattened responses.
Agreed. I'm okay with the fix that @jabley has contributed in https://github.com/alphagov/backdrop/commit/f6870d334313a4fe2e2b74f12e843e30ef0c52c3 here though