Fixed rollback of extra_data migration

[niklas-scholz]

Proposed changes

The extra_data json field migration 0013_migrate_extra_data.py was not revertable since the is_text_field type check was throwing a TypeError since the first argument is an instance instead of a class. This error occurs only when reverting migrations on databases other than SQLite.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.72%. Comparing base (fb93814) to head (6769fb7).
Report is 62 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #626   +/-   ##
=======================================
  Coverage   91.72%   91.72%           
=======================================
  Files          39       40    +1     
  Lines        1172     1185   +13     
  Branches      144       77   -67     
=======================================
+ Hits         1075     1087   +12     
- Misses         72       73    +1     
  Partials       25       25           

Flag	Coverage Δ
unittests	91.72% <ø> (+<0.01%)	⬆️

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[nijel]

Merged, thanks for your contribution!