Add Author dropdown to PostSettingsMenu

Ref #3084
This PR does NOT hide the dropdown as required to close 3084.

- `EditorNewRoute` creates post with the author set to the current user
- added `authors` ArrayPromiseProxy (whoa, what?) to PSM
- added `changeAuthor` function that sets author and saves model when the user selects a new author
Merge pull request #3324 from novaugust/psm-authors

Add Author dropdown to PostSettingsMenu
Pagination on the Users Management screen

closes #3222
- implementing server-side pagination for /users API
- passing /users?limit=none will return all users
- passing /users?status=invited will filter base on user status
- creating 3 new mixins (route, controller and view) to encapsulate pagination functionality
- updating route, controller and view for Posts to use new mixing
- implementing infinite scrolling on the Users Management screen (using new mixins)
- Users Management screen now displays all invited users, but paginates active users