[common][serializers] Fix object existence check

The check worked with SQLite but not with PostgreSQL.
In any case, since we have a user object here, there's no
reason to not use it directly.

beat/web/common/serializers.py

@@ -351,7 +351,7 @@ class ContributionCreationSerializer(serializers.ModelSerializer):
             )
 
         if self.Meta.model.objects.filter(
-            author__username__iexact=user, name=name, version=version
+            author=user, name=name, version=version
         ).exists():
             raise serializers.ValidationError(
                 "{} {} version {} already exists on this account".format(