Fix storage helpers

[SchrodingersGat]

• Remove os.path reliance
• Move to standard django accessors

[netlify]

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Name	Link
🔨 Latest commit	f85d493
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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.46%. Comparing base (91bf761) to head (f85d493).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #11697      +/-   ##
==========================================
+ Coverage   91.45%   91.46%   +0.01%     
==========================================
  Files         965      965              
  Lines       49990    49989       -1     
==========================================
+ Hits        45717    45723       +6     
+ Misses       4273     4266       -7     

Flag	Coverage Δ
backend	89.27% <100.00%> (+0.01%)	⬆️

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Components	Coverage Δ
Backend Apps	91.73% <ø> (ø)
Backend General	93.41% <ø> (ø)
Frontend	∅ <ø> (∅)

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