Stripe endpoint improvements

[LMNTL]

Checklist

1. If you've added code that should be tested, add tests
2. If you've changed APIs, update (or create!) the documentation
3. Ensure the tests pass
4. Make sure that your code lints and that you've followed our coding style
5. Write a title and, if necessary, a description of your work suitable for publishing in our release notes
6. Mention any related issues in this repository (as #ISSUE) and in other repositories (as kobotoolbox/other#ISSUE)
7. Open an issue in the docs if there are UI/UX changes

Description

Stripe checkout now collects billing address info.

Notes

Non-user-facing improvements:

• The Customer name and billing address in Stripe are now updated after checkout based on user input.
• Customer metadata is now updated on every checkout instead of only when creating a Customer.
• Adds the user ID, username, and domain to newly created Customer and Subscription objects in Stripe for easier reporting.
• Caches the Products endpoint per-request.