[#4330] Add unique constraint to group configs names

[Viicos]

Partly closes #4330 (in preparation of using natural keys.

Checklist

Check off the items that are completed or not relevant.

• Impact on features

  • Checked copying a form
  • Checked import/export of a form
  • Config checks in the configuration overview admin page
  • Problem detection in the admin email digest is handled

• Release management

  • I have labelled the PR as "needs-backport" accordingly

• I have updated the translations assets (you do NOT need to provide translations)

  • Ran ./bin/makemessages_js.sh
  • Ran ./bin/compilemessages_js.sh

• Commit hygiene

  • Commit messages refer to the relevant Github issue
  • Commit messages explain the "why" of change, not the how

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.57%. Comparing base (b0feb8a) to head (89cdff6).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4532   +/-   ##
=======================================
  Coverage   96.57%   96.57%           
=======================================
  Files         719      719           
  Lines       24077    24077           
  Branches     2862     2862           
=======================================
  Hits        23253    23253           
  Misses        560      560           
  Partials      264      264           

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[sergei-maertens]

I think adding a unique slug field makes more sense than making names unique? My gut feeling says that names will not be a stable identity between versions of forms 🤔

This is also a breaking change, so a detection script for non-unique slugs would need to be added to make them fix the data before upgrading.

[sergei-maertens]

Going to put this one back in draft so I stop looking at it!