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

lays groundwork for #2702

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.18%. Comparing base (6096098) to head (0855103).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3099   +/-   ##
=======================================
  Coverage   70.18%   70.18%           
=======================================
  Files         213      212    -1     
  Lines        6819     6799   -20     
=======================================
- Hits         4786     4772   -14     
+ Misses       2033     2027    -6     

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