Filter folders' queryset to domains

[nas-tabchiche]

No description provided.

[eric-intuitem]

We need to be able to put assets and threats either in a domain or in the root folder (seen as "Global").
For other objects created by a user, they should be in a domain.
I have updated the architecture accordingly.

[nas-tabchiche]

Creating a proxy model Domain, in the same fashion as Policy could save us a bit of headache here. This will make the distinction between domain and folder clearer in the codebase, and leave no room for confusion. I will do this first thing in the morning tomorrow

[eric-intuitem]

Simpler spec: anything put in the global folder has is_published=True. Only projects cannot be attached to the global folder, which is already implemented.

[eric-intuitem]

LGTM