Fix data migration

[tumb1er]

The problem with overriden AssetType model is that migration dependency is broken:

• formats data migration in image_assets must be applied before deletion of format field in assets
• formats field must be applied after adding formats field in assets

While first dependency could be established in assets migration, second dependency belongs to image_assets library and could not be set up properly.
The solution is to disable data migration for abstract AssetType model in favor of copying this migration to an assets application.

[codecov]

Codecov Report

Merging #35 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #35   +/-   ##
=======================================
  Coverage   97.85%   97.85%           
=======================================
  Files          24       24           
  Lines         652      653    +1     
=======================================
+ Hits          638      639    +1     
  Misses         14       14           

Flag	Coverage Δ
#unittests	97.85% <100.00%> (+<0.01%)	⬆️

Impacted Files	Coverage Δ
image_assets/migrations/0006_migrate_formats.py	76.92% <100.00%> (+1.92%)	⬆️

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