DMN config checks / digest email

[sergei-maertens]

Closes #4284

Changes

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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