migrations: optimize extra_data migration

[nijel]

Proposed changes

This makes it more friendly to huge databases.

Fixes #501

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[codecov]

Codecov Report

Attention: 8 lines in your changes are missing coverage. Please review.

Comparison is base (a8713fa) 94.12% compared to head (36c3e59) 93.63%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #520      +/-   ##
==========================================
- Coverage   94.12%   93.63%   -0.49%     
==========================================
  Files          39       39              
  Lines        1140     1147       +7     
  Branches      136      138       +2     
==========================================
+ Hits         1073     1074       +1     
- Misses         43       48       +5     
- Partials       24       25       +1     

Flag	Coverage Δ
unittests	93.63% <11.11%> (-0.49%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
...ocial_django/migrations/0013_migrate_extra_data.py	29.41% <11.11%> (-3.93%)	⬇️

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