Orphan documents

Deleting a user who is the owner of documents causes 500 internal server error on project pages, could be the same for tags.

- Add a new ```UserNullable``` concern to be include in any association between and user and that particular entity; the concern will return a ```NullUser``` if a nil one is set.

https://www.pivotaltracker.com/n/projects/982890/stories/66066822
I would not recommend implementing `method_missing`, it will prevent the class from raising errors on genuine bugs. The gem provides a clean solution to the issue, I would prefer an implementation similar to that.