Add support for interactive notifications

[dreamflasher]

Please add support for interactive notifications, allowing users to see the message content and directly replying, for example from the Apple watch.

[chrisballinger]

I laid the foundation for this in v4.0 with our support for the iOS 10 notifications API. I started the implementation, and it's theoretically pretty close, but will require some refactoring to move our message sending logic out of the messages view controller.