[common][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/common/serializers.py

@@ -364,7 +364,7 @@ class ContributionModSerializer(serializers.ModelSerializer):
 
 
 class ContributionCreationSerializer(ContributionModSerializer):
-    fork_of = serializers.JSONField(required=False)
+    fork_of = serializers.CharField(required=False)
     previous_version = serializers.CharField(required=False)
     version = serializers.IntegerField(min_value=1)