User managment screen, first pass

Fixes #3078
- new "users" route, with matching controller and template
- fetching real data from /ghost/api/v0.1/users/
- updated "user" route to accept a :user_id as a URL parameter
- updated labels everywhere (from "user" to "users")
- updated "profile" link to header to point to proper "users/:user_id" route
- updated broken test cases related to this screen