Migrate carrierwave to activestorage

[kkoehn]

• Avoid saving attachment when file content gets saved in content
• look for usages of native_file and continue migration

[codecov]

Codecov Report

❌ Patch coverage is 40.00000% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.05%. Comparing base (d248f40) to head (78e6b5b).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
app/controllers/code_ocean/files_controller.rb	16.66%	5 Missing ⚠️
app/controllers/exercises_controller.rb	50.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3099      +/-   ##
==========================================
- Coverage   70.24%   70.05%   -0.20%     
==========================================
  Files         215      214       -1     
  Lines        6850     6826      -24     
==========================================
- Hits         4812     4782      -30     
- Misses       2038     2044       +6     

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