Add an API to return suggested product type choices

[jwalgran]

Overview

We allow contributors to submit any value for product type but expect that by suggesting standard values and values that have already been submitted we will have more standardization in the collected data.

We use a helper function so we can share the logic with a public API and with the claim form serializer which was already pulling a list of values. Now that we have migrated claims to use extended fields we can replace the old logic used to look for product types used on other claims.

Connects #1706

Demo

Testing Instructions

• Verify that the new product-types API is available at http://localhost:8081/api/docs/#!/product-types/product_types_list and that it returns a list of unique types.
• Log in as c2@example.com and submit a claim
• In a separate browser log in as c1@example.com, browse http://localhost:8081/dashboard/claims/ and approve the claim
• In the original session browse http://localhost:6543/claimed, edit the claim, and verify that the product types dropdown lists is filled with values.

Checklist

• fixup! commits have been squashed
• CI passes after rebase
• CHANGELOG.md updated with summary of features or fixes, following Keep a Changelog guidelines

[jwalgran]

There is a test of the claim form product types that I will fix up

[emilyhu0106]

This endpoint works really well, especially the helper function that works for both claim form and public API

[jwalgran]

Thanks for the review.