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
We are using Time.parse in another place, but even in there is not correct,
because Time.current is what active support use to calculate 5.days.ago
type of methods, so that's not been mocked by Time.now
On Aug 26, 2014 9:44 AM, "Yaro" <notifications@github.com> wrote:

> In spec/models/hangout_spec.rb:
>
> > @@ -21,7 +21,7 @@
> >      before { hangout.hangout_url = 'test' }
> >
> >      it 'reports live if the link is not older than 5 minutes' do
> > -      allow(Time).to receive(:now).and_return(Time.parse('10:04:59'))
>
> I see. We are using Time.parse('10:04:59 UTC') in other places
>
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> <https://github.com/AgileVentures/WebsiteOne/pull/513/files#r16713519>.
>