Migrate carrierwave to activestorage

[kkoehn]

We should consider the following points:

• Migration of native_files to ActiveStorage
• Removal of native_file column
• I removed CodeOcean::File#hash_content since I don't think it was used. If it is indeed unused, the column could be removed as well

[codecov]

Codecov Report

❌ Patch coverage is 80.76923% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.13%. Comparing base (d248f40) to head (f688624).
⚠️ Report is 13 commits behind head on main.

Files with missing lines	Patch %	Lines
app/controllers/code_ocean/files_controller.rb	50.00%	3 Missing ⚠️
app/models/code_ocean/file.rb	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3099      +/-   ##
==========================================
- Coverage   70.24%   70.13%   -0.12%     
==========================================
  Files         215      214       -1     
  Lines        6850     6833      -17     
==========================================
- Hits         4812     4792      -20     
- Misses       2038     2041       +3     

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