fix: make migration 0013 reversible

[browniebroke]

Proposed changes

As mentioned #495, the migration isn't reversible. I don't think we want to change anything in the backwards operation, so we might be enough to use RunPython.noop.

Write a function to set the old fields from the _new JSON fields.

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Codecov Report

Attention: 24 lines in your changes are missing coverage. Please review.

Comparison is base (e90019f) 93.63% compared to head (cc31ccc) 91.72%.

Files	Patch %	Lines
...ocial_django/migrations/0013_migrate_extra_data.py	7.69%	24 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #533      +/-   ##
==========================================
- Coverage   93.63%   91.72%   -1.92%     
==========================================
  Files          39       39              
  Lines        1147     1172      +25     
  Branches      138      144       +6     
==========================================
+ Hits         1074     1075       +1     
- Misses         48       72      +24     
  Partials       25       25              

Flag	Coverage Δ
unittests	91.72% <7.69%> (-1.92%)	⬇️

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[nijel]

How is noop better here than failing? It just hides the fact that the data are not migrated during the reverse migration, what could lead to data loss.

[browniebroke]

What kind of data loss are we talking about?

[browniebroke]

Ah I see, we have the data being moved over from an old field to a new field. I get the problem you're raising now.

[nijel]

Merged, thanks for your contribution!