Add managing editors role

Tracker: https://www.pivotaltracker.com/story/show/58707108

Adds a new "Managing Editor" role, which has the same permissions as a departmental editor, plus the ability to unpublish/archive editions. Note that GDS Editors lose this permission at the same time.

This will probably need a rebase after the archiving story is merged because of the changes to the cucumber features, but the main code changes shouldn't need to change.