Add private_data field to ExtraUserDetail

[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
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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

Added 'private_data' field to the extra user detail model.

Notes

There are some properties that need to be set on the User but not editable/viewable from the /me endpoint. As a concrete example, for mass emails, we use a boolean in extra_details to store whether a particular email has been sent to a user. We don't want users to be able to unset that boolean, because it might result in them getting sent the same email twice.