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/postgres_tests/fields.py

@@ -10,7 +10,7 @@ try:
     from django.contrib.postgres.fields import (
         ArrayField, BigIntegerRangeField, CICharField, CIEmailField,
         CITextField, DateRangeField, DateTimeRangeField, DecimalRangeField,
-        HStoreField, IntegerRangeField, JSONField,
+        HStoreField, IntegerRangeField,
     )
     from django.contrib.postgres.search import SearchVectorField
 except ImportError:
@@ -26,10 +26,6 @@ except ImportError:
             })
             return name, path, args, kwargs
 
-    class DummyJSONField(models.Field):
-        def __init__(self, encoder=None, **kwargs):
-            super().__init__(**kwargs)
-
     ArrayField = DummyArrayField
     BigIntegerRangeField = models.Field
     CICharField = models.Field
@@ -40,7 +36,6 @@ except ImportError:
     DecimalRangeField = models.Field
     HStoreField = models.Field
     IntegerRangeField = models.Field
-    JSONField = DummyJSONField
     SearchVectorField = models.Field