Change Article factory to more closely match app behavior.

Articles are written by users on behalf of organizations; the user
factory has an organization association, but the organization factory
doesn't by default have a user association.  Changing the article
factory's organization association to a user association covers both
without disrupting the existing test suite.
Add a scope to filter out deactivated users

That is, users that themselves have been deactivated, or users who
belong to deactivated organizations.
https://www.pivotaltracker.com/story/show/44384531
Update article feature

Reflects new implementation of Article factory association.  See
30b3622.
Revert "Add a scope to filter out deactivated users"

Katy asked for deactivated orgs to have their stuff hidden; she didn't
say anything about users, and I can imagine the majority of situations
in which users are deactivated might want their articles and etc. to be
retained.  So, revert this.
This reverts commit 4f865c79b40468cf28bad7bc9716ab26a5ee9510.