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
I would like some feedback specifically on this new property: the goal is to allow for modals to have a custom wrapper class (if they need it) to make styling specific modals easier.
Hopefully their aren't any objections and the property name is meaningful enough