Support product data (and dicts)
Handle product_data in import/export
Switch format of outputted fixture data

From:

<product_data>
  <data key="num">1234</data>
</product_data>

To:

<product_data>
  <num>1234</num>
</product_data>
Make product_data a real model property
Only add product_data node if there is data
Add test for product_data in fixture
Add UI for product_data
Cleanup
Put custom product data behind feature flag
Merge pull request #3622 from dimagi/product_data

Custom product data