Move user notifications out of NavigationViewController

[1ec5]

NavigationViewController is currently responsible for creating and scheduling user notifications (push notifications) on the lock screen that are synchronized with spoken instructions. We should move the user notification functionality into its own component so that applications with custom UIs don’t have to roll their own user notifications.

This would also be a good opportunity to make user notifications optional: #1957.

/cc @mapbox/navigation-ios @d-prukop