[refactor] Attachment images

[SchrodingersGat]

This PR adds / refactors some functionality related to attachments

Changes

• Adds "thumbnail" for attachments which are images
• Display thumbnail in attachment table
• Prevent upload of files above a user-configurable size
• Data migration for existing uploaded attachments

Rationale

This forms the first part of a re-think of how we might approach generic images against any model instance - see #10484

However, none of that will be implemented here - this is a low-scope change to allow discrimination of attachments which are also images, and display them as such in the UI

Tasks

• Offload thumbnail generation to the background worker
• Database migration unit tests
• Demo dataset migration
• Unit tests for new features

Future Work (Potentially, TBD)

• Replace part / company "image" fields with standard attachments - [FR] Multiple Part Pictures #9788
• The "primary" image associated with an instance is just an uploaded attachment
• Remove django-std-image entirely - [FR] Deprecating django-stdimage and migrate to django-picture #11647
• Remove django_cleanup entirely - handled internally
• Remove the "select part image from existing" functionality - in favor of generic attached images

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[codecov]

Codecov Report

❌ Patch coverage is 92.74611% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.31%. Comparing base (9cac925) to head (0f79a27).
⚠️ Report is 1 commits behind head on master.

❌ Your patch check has failed because the patch coverage (92.70%) is below the target coverage (95.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #11961      +/-   ##
==========================================
- Coverage   91.44%   91.31%   -0.14%     
==========================================
  Files         974      969       -5     
  Lines       50704    50074     -630     
==========================================
- Hits        46366    45724     -642     
- Misses       4338     4350      +12     

Flag	Coverage Δ
backend	90.32% <89.63%> (+1.05%)	⬆️
migrations	40.69% <45.07%> (-1.03%)	⬇️

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Components	Coverage Δ
Backend Apps	91.60% <ø> (-0.17%)	⬇️
Backend General	93.37% <92.70%> (-0.12%)	⬇️
Frontend	∅ <ø> (∅)

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[matmair]

LGTM overall; if we want to change attachments meaningfully in 1.4.0 I would have some larger points worth adressing