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
well the `NullUser` should behave exactly like a normal `User`, the easiest way to achieve this is to subclass the model itself. But maybe you are right, we don't need all that functionality at the moment, although it doesn't hurt and saves us from headaches in the future.