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
UCLA reports: Patient target dates calculations will be processed by signals now.
remove references to hqbilling in corehq
remove HQBillingAddress and HQBillingMixin
delete payments submodule
remove reference to HQBillingMixin in domain doc
Make commtrack-system cases default
truly uniquify names
preserve transparency in png files
Merge pull request #3622 from dimagi/product_data

Custom product data
Merge pull request #3633 from dimagi/default-commtrack-cases

Make commtrack-system cases default
Merge pull request #3636 from dimagi/multimedia-transparent-png

preserve transparency in png files