Add Active concept to user

Users can be de-activated by admins.  Also future support of de-
activating
a user when their Preservation Network org is deleted.
Merge branch 'master', remote-tracking branch 'origin' into topic/user-deactivation
Delete an org will deactive org users

Also, deleting now just deactivates the Org so you can still see the
reference in comments
Merge pull request #67 from azavea/topic/org-delete

Improve Organization deletion
Merge pull request #64 from azavea/topic/user-deactivation

Add `active` concept to users