Fixed #12990, Refs #27694 -- Added JSONField model field.

Thanks to Adam Johnson, Carlton Gibson, Mariusz Felisiak, and Raphael
Michel for mentoring this Google Summer of Code 2019 project and
everyone else who helped with the patch.

Special thanks to Mads Jensen, Nick Pope, and Simon Charette for
extensive reviews.

Co-authored-by: Mariusz Felisiak <felisiak.mariusz@gmail.com>

tests/queries/models.py

@@ -747,3 +747,10 @@ class ReturningModel(models.Model):
 
 class NonIntegerPKReturningModel(models.Model):
     created = CreatedField(editable=False, primary_key=True)
+
+
+class JSONFieldNullable(models.Model):
+    json_field = models.JSONField(blank=True, null=True)
+
+    class Meta:
+        required_db_features = {'supports_json_field'}