Custom product data

This PR is for merging custom product data into the custom user data branch (https://github.com/dimagi/commcare-hq/pull/4292) that it was branched off of. Ultimately these are going live together, but this PR is being made to make review easier.
i think i already made this comment on the other branch, but would be nice if you could just instantiate a `CustomDataFieldsView` with params instead of having to create a class that's really just a `CustomDataFieldsView` configured with 4 things