Stripe endpoint tests

[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 tests for the Stripe checkout link and customer portal API endpoints. Also fixes some issues with the endpoints discovered while getting tests passing.

Related issues

[bufke]

Is the idea here that we actively want it to error when no price exists? I don't think I have an opinion between a GET with 404 vs POST. I'm not sure I like this uncaught DoesNotExist possibility. An end user could not understand our API and send random price ids that don't exist. That should generate a 4xx level error. I could see some justification to log a warning to help get our attention but I don't see it as an error if the end user of the API can generate it.

It could be solved by adjusting the serializer to actually reference the Price by id instead of just a CharField. Unsure if this would make the code easier to read or not.

Not a blocker but let's change or discuss this.