Update endpoints to handle Enterprise plans

[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

Update endpoints to allow managing Enterprise plans. Update the service usage endpoint to get stats for all users in an organization if they have an Enterprise plan .

Notes

Mostly small changes - CustomerPortalView needed to be updated to handle the fact that Enterpise plans and storage add-ons aren't included in the default billing configuration. Also, the check in /organizations/{org id}/service_usage/ for all users in the current org has been uncommented, bugfixed, and made to only apply if the user has an Enterprise plan (Gold, Platinum.)

Related issues

Front-end changes are included in kpi#4782

[bufke]

Don't forget the unit test 😄

[notion-workspace]

Uncomment code to check org for all users in org

[bufke]

This file merits broader review. Let's call it out of scope of this PR and we can change it later too. I'd love to discuss consistent generation of test data.

[bufke]

GitHub requires me to type here

[bufke]

LGTM. Is this the right branch to merge to? Do we still need to test it more on a staging site since changes were made?