UX improvements: modal registration forms for email and stuf

[robinmolen]

Closes #4686

Changes

The registration forms for Email and StUF-ZDS now use React components, instead of react-json-schema-form generated forms. This allows us to share styling and keep the styling uniform across forms. These changes also improve the user experience, by providing a simular form layout/logic as the other registration inline forms.

Checklist

Check off the items that are completed or not relevant.

• Impact on features

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

• Release management

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

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

  • [X] Ran ./bin/makemessages_js.sh
  • [X] Ran ./bin/compilemessages_js.sh

• Commit hygiene

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

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.

Please upload report for BASE (master@8f11d28). Learn more about missing BASE report. Report is 19 commits behind head on master.

Files with missing lines	Patch %	Lines
...openforms/registrations/contrib/stuf_zds/plugin.py	80.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #4742 +/- ## ========================================= Coverage ? 96.55% ========================================= Files ? 748 Lines ? 25410 Branches ? 3358 ========================================= Hits ? 24535 Misses ? 610 Partials ? 265 ```

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