[experiments][serializers] Fix fork_of field type

It was a JSONField however it should be a CharField.
It worked because the DRF JSONField does not actually
do type checks so a string can be dumped and it passes
the validation successfully.

beat/web/experiments/serializers.py

@@ -54,7 +54,7 @@ from .models import Experiment, Block
 class ExperimentCreationSerializer(serializers.ModelSerializer):
     declaration = beat_fields.JSONField()
     description = serializers.CharField(required=False, allow_blank=True)
-    fork_of = beat_fields.JSONField(required=False)
+    fork_of = serializers.CharField(required=False)
     toolchain = serializers.CharField()
 
     class Meta: