735 owner cant leave project

closes #735 
I do not think this is the appropriate way to use serializers, I would have a `RemoveUser[FromProject]Serializer`, and handle the remove functionality on the `save` method. Depending on expected inputs for the serializer, I will have `_validate` methods which will perform proper checks, incase of failure they would raise some `ValidationError` exceptions. In this case it becomes as simple as:
```python
serializer = RemoveUser[FromProject]Serializer(request.DATA)
if serializer.is_valid():
    serializer.save()
    return Response(serializer.data)  # or a 204 with no content to indicate success.
else:
    return Response(data=serializer.errors)
```
Also I would deal with this through a `DELETE` request.