Use NLPUsageCounter date field to fix User Statistics report

[JacquelineMorrissette]

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
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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

Fix the superusers User statistics report

Notes

Adding the date field to the NLPUsageCounter broke the report, this PR fixes it :)

[LMNTL]

Thank you for fixing that! It works in testing.