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
Works for me, although it seems like the `gh-modal-dialog.js` component should have those bits built into it as it generates a wrapper element anyways, right? Then again, if it's not creating the modal-container, it must have another wrapper built on top of it... want to double check that it's not emitting a useless, unneeded div?  If so, you should move the `#modal-container` into the component's code, rather than its template