[#4267] Add management command to migrate existing configurations

[Viicos]

Originally for #4267

Relates to #4541 (although I think this can be merged independently)

Changes

Add a management command to migrate V1 and V2 configurations.

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

Attention: Patch coverage is 84.05797% with 11 lines in your changes missing coverage. Please review.

Project coverage is 96.56%. Comparing base (a0e2e18) to head (468a8af).
Report is 622 commits behind head on master.

Files with missing lines	Patch %	Lines
.../management/commands/migrate_objects_api_config.py	82.53%	8 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4536      +/-   ##
==========================================
- Coverage   96.60%   96.56%   -0.04%     
==========================================
  Files         725      726       +1     
  Lines       24125    24197      +72     
  Branches     2870     2885      +15     
==========================================
+ Hits        23305    23366      +61     
- Misses        559      567       +8     
- Partials      261      264       +3     

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[Viicos]

Not sure if the remaining coverage bits should be properly addressed

[sergei-maertens]

Closing this - the way we're adding the new functionality in a backwards compatible way relieves the pressure for a migration tool.