Make default admin filter scoped to org only

This removes the previous role-specific functionality and replaces it
with a single piece of logic that defaults to all active documents in
the user's organisation.

https://www.pivotaltracker.com/story/show/56207006
Merge pull request #845 from alphagov/default-admin-filters

https://www.pivotaltracker.com/story/show/56207006

Make default admin filter scoped to org only