Fix/unassign user

[eaglesemanation]

I want to merge this change to fix #498

PR intended to be tested with API branch: master

Screenshots

• Before:
  

• After:
  

Pull Request Checklist

1. All visible strings are translated with proper context.
2. All data-formatting is locale-aware (dates, numbers, and so on).
3. Translated strings are extracted.
4. Number of API calls is optimized.
5. The changes are tested.
6. Data-test are added for new elements.
7. Type definitions are up to date.
8. Changes are mentioned in the changelog.

Test environment config

API_URI=https://master.staging.saleor.rocks/graphql/

[krzysztofwolski]

@eaglesemanation Thank you! :)