Usage limits release fixes

[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

Fixes a bug where users on the Professional plan were showing usage limits from the Legacy plan.

Notes

1. Refactors getAccountLimits() so FREE_TIER_THRESHOLDS only applies to users without a plan
2. Fixes FREE_TIER_THRESHOLDS not being applied to eligible users (see here)
3. Changes response.results[0]?.plan?.product to response.results[0]?.items[0]?.price?.product in onFetchSubscriptionInfoDone()
4. Removes stopgap that prevents warnings/modals from showing
5. Prevent usage banners/modals displaying when they shouldn't by refactoring useExceedingLimits to use getAccountLimits()

[jnm]

🎉