renamed 'requires_checks' -> 'requires_system_checks' + deleted BaseCommand._get_requires_checks method
added test for CheckMessage.is_error
added test for DatabaseValidation.validate_field
deleted unused check for referring to inherited field
small cleanup: deleted dead if
deleted unused check
extracted common part of {GenericForeignKey, GenericRelation}._check_content_type_field methods
small doc improvement
added test_valid_foreign_key_without_accessor
changed: "Model.field.a_field" -> "Model._meta.get_field('a_field')"
deleted unused filter