Make timestamp representation consistent in model string representation.

[sergei-maertens]

Closes #3705

Changes

I've grepped the project for def __str__(self): and checked all of them for their timestamp usages.

• Removed the custom __str__ from SubmissionStep, as it adds no value. As a counter point, the timestamps are now included in the admin inline.
• Updated the timestamps that do make sense to ensure they're localized and using the localtime representation
• Cleaned up SubmissionStep and removed the obsoleted _data JSONField, which made it possible to use a tabular inline and that made it possible to display the timestamps in their own table columns without cluttering the submissions admin too much.

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 2 lines in your changes missing coverage. Please review.

Project coverage is 96.56%. Comparing base (1ae8370) to head (3e1e318). Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
src/openforms/forms/models/form_version.py	50.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #4749 +/- ## ========================================== - Coverage 96.56% 96.56% -0.01% ========================================== Files 746 746 Lines 25187 25191 +4 Branches 3313 3313 ========================================== + Hits 24322 24325 +3 - Misses 601 602 +1 Partials 264 264 ```

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[sergei-maertens]

• https://github.com/open-formulieren/open-forms/blob/master/src/openforms/forms/models/form_version.py#L94

oh I missed that one! I'll update