Implementation of "invite a new user" modal

Closes #3079
- new controller and template for invite-new-user-modal
- actually triggers email invite via POST /ghost/api/v0.1/users/
- setting default language value (on the client) when creating a user
- only available role is "Author"
- updates to UsersIndexController to allow dynamic property calculation and template rending
One small request, from a styling point of view: Could you add `class="invite-new-user"` to the modal?