Chore: upgrade to DRF 3.15

[shamoon]

Proposed change

Not the prettiest, but workaround:

• 3.15 regression: Serializer validation failed for unique together constraint  encode/django-rest-framework#9358
• Issue with upgrade from 3.14 to 3.15.1, null values no longer allowed in POST encode/django-rest-framework#9410

Welcome any feedback, of course.

Closes #(issue or discussion)

Type of change

• Bug fix: non-breaking change which fixes an issue.
• New feature / Enhancement: non-breaking change which adds functionality. Please read the important note above.
• Breaking change: fix or feature that would cause existing functionality to not work as expected.
• Documentation only.
• Other. Please explain:

Checklist:

• I have read & agree with the contributing guidelines.
• If applicable, I have included testing coverage for new code in this PR, for backend and / or front-end changes.
• If applicable, I have tested my code for new features & regressions on both mobile & desktop devices, using the latest version of major browsers.
• If applicable, I have checked that all tests pass, see documentation.
• I have run all pre-commit hooks, see documentation.
• I have made corresponding changes to the documentation as needed.
• I have checked my modifications for any breaking changes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.30%. Comparing base (064d384) to head (8cebe8c).

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #7134      +/-   ##
==========================================
+ Coverage   97.28%   97.30%   +0.01%     
==========================================
  Files         443      443              
  Lines       18479    18484       +5     
  Branches     1473     1562      +89     
==========================================
+ Hits        17978    17985       +7     
+ Misses        501      496       -5     
- Partials        0        3       +3     

Flag	Coverage Δ
backend	96.04% <100.00%> (+0.02%)	⬆️
frontend	98.67% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.