notifications (#10), add /interaction/invite, lanternFriends modal improvements

- specify notifications
  - new top-level "notifications" field in model: array of strings
  - allow closing a notification via /interaction/close with {notification: <index>}
  - (backend can automatically close a given notification after a timeout is
    reached if the user has not closed it already. mock backend does not implement this though)
- new /interaction/invite API; payload: array of email addresses
  - /interaction/continue on lanternFriends modal no longer possibly sends
    invites, as we now have a separate interaction for this
- separate "send invites" button, style improvements (always show scrollbars in
  horizontal contact lists in (mountain) lion