Full iOS push notification support

[MartinBriza]

There's some rudimentary support now for opening a buffer number from notification data but I foresee this will be an issue if the app was not running when the notification has been clicked and relying on buffer numbers is icky. It'd be better to use buffer pointers. Also, NOT showing notifications for buffer that's currently open when the app is in the foreground would be nice.

For users: This feature won't be generally available for a while because it needs server support and I need to figure out a sensible way to provide this.