Implement storage- and relationship-aware cleanup

[Swatinem]

This implements a generic mechanism to resolve the model relationship graph based on Django metadata. That graph is then sorted in topological order, and all models are being deleted. Special manual deletion code is written for all the models that have some files in storage, in order to make sure that files are also being cleaned up properly.

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This is work towards codecov/engineering-team#1127

[codecov-notifications]

Codecov Report

Attention: Patch coverage is 30.00000% with 98 lines in your changes missing coverage. Please review.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
services/cleanup/models.py	0.00%	68 Missing ⚠️
services/cleanup/cleanup.py	0.00%	17 Missing ⚠️
services/cleanup/utils.py	0.00%	8 Missing ⚠️
services/cleanup/relations.py	87.50%	5 Missing ⚠️

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[github-actions]

✅ All tests successful. No failed tests were found.

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[codecov]

Codecov Report

Attention: Patch coverage is 30.00000% with 98 lines in your changes missing coverage. Please review.

Project coverage is 97.70%. Comparing base (dc51116) to head (389d49e).
Report is 2 commits behind head on main.

❗ Current head 389d49e differs from pull request most recent head 73f9f21

Please upload reports for the commit 73f9f21 to get more accurate results.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
services/cleanup/models.py	0.00%	68 Missing ⚠️
services/cleanup/cleanup.py	0.00%	17 Missing ⚠️
services/cleanup/utils.py	0.00%	8 Missing ⚠️
services/cleanup/relations.py	87.50%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #973      +/-   ##
==========================================
- Coverage   97.96%   97.70%   -0.27%     
==========================================
  Files         446      451       +5     
  Lines       36045    36185     +140     
==========================================
+ Hits        35311    35353      +42     
- Misses        734      832      +98     

Flag	Coverage Δ
integration	41.88% <5.00%> (-0.15%)	⬇️
unit	90.40% <30.00%> (-0.24%)	⬇️

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