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
my initial idea was to avoid duplicating some of the computational methods in the class such as `display_name` and `full_name`. Although you are right, the entire functionality is not required and at the moment, removing it will cause no harm. But if it is possible to use a gem or a custom solution to easily allow the class to respond to the right methods, then I am all for it. I just don't want to have to define methods every time we see an `undefined method for ...` error :wink: