cleanup: renamed d -> context
changed custom user model checks -- raise Warning instead of an Error
changed formatting errors
split django/core/checks/base.py into smaller files
renamed: checking framework -> [system] check framework
fixup in ManyToManyField._check_relationship_model -- checking missing through model
added skipping autocreated through models
added skipping automatically created through models in ManyToManyfield._check_relationship_model
added underscore prefix to MISSING_MODEL_MESSAGE
deleted unused imports (get_models)
merged _check_on_delete_set_{null, default}
moved on_delete checks from RelatedField to ForeignObject
added newline at end of docs/ref/checks.txt
added valid charfield test
using skipIfDBFeature in RelativeFieldTests.test_nullable_primary_key
polishing invalid_models tests: better test names, spliting OtherFieldTests into smaller classes
introduced message levels
fixup for 'changed mysql field validation...'