closes#3352
- adding a ```type``` attribute to buttons inside form in the settings section
- scanning the rest of the project to find any other buttons w/a missing attribute
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" - pending 3196
- updates to UsersIndexController to allow dynamic property calculation and template rending
