require user names in registration changeset

[midigofrank]

Notes for the reviewer

Currently user names are required in the user registration form, but this was not enforced in the changeset.

Related issue

Paves way for: https://github.com/OpenFn/thunderbolt/issues/125

Review checklist

• I have performed a self-review of my code
• I have verified that all appropriate authorization policies (:owner, :admin, :editor, :viewer) have been implemented and tested
• If needed, I have updated the changelog
• Product has QA'd this feature

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.12%. Comparing base (2cdd274) to head (10b436f).
Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2068   +/-   ##
=======================================
  Coverage   90.12%   90.12%           
=======================================
  Files         252      252           
  Lines        8261     8261           
=======================================
  Hits         7445     7445           
  Misses        816      816           

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