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
@yggie @YaroSpace I won't do the change to the ```NullUser``` in this PR I think we should give it a little bit more of thought. In case you guys don't think adding a new gem is a good idea we can always just implement our own null object without the inheritance, which will only cause a nil pointer exception at some point.