Display roles, pagination on the Users Management screen and necessary ```/users``` API changes

closes #3222, closes #3287, closes #3085
- /users/me API  now returns roles by default
- displaying a users roles on the Users Management screen
- creating 3 new mixins (route, controller and view) to encapsulate pagination functionality
- updating route, controller and view for Posts to use new mixing
- implementing server-side pagination for /users API
- implementing infinite scrolling on the Users Management screen (using new mixins)
- Users Management screen now displays all invited users, but paginates active users
Updating test cases to reflect user pagination data