Add success message after Stripe checkout

[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 description of your work suitable for publishing on our forum
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

Adds a success popup when a user returns from a successful transaction at Stripe.

Related issues

Notes

Will return a message telling the user to wait a few minutes and refresh the page if their subscription hasn't yet been updated locally.

This doesn't make any API calls, it just sets the price ID in the return URL when we generate the checkout link. Then when the user comes back from Stripe, it checks that price ID against the list of current subscriptions. I had to pull in some unmerged changes from feature/billing to test it properly.

Screenshot

[bufke]

My change requests are minor, please proceed to merge once you review them.