Disallow rotation for onboarding

Assumptions:

• You're using UIPageViewController for onboarding
• You have an UIViewController with a container that embeds the UIPageViewController
• You present this UIViewController modally

The UIViewController container lets you do two things:

• Add a close button that's always visible, and doesn't scroll with the UIPageViewController
• Prevent rotation to landscape
• Rotate to portrait if the user was already in landscape to begin with

// UIViewController

override func viewWillAppear(animated: Bool) {
  if UIDeviceOrientationIsLandscape(UIDevice.currentDevice().orientation) {
    let value = UIInterfaceOrientation.Portrait.rawValue
    UIDevice.currentDevice().setValue(value, forKey: "orientation")
  }
}

override func shouldAutorotate() -> Bool {
    return !UIDeviceOrientationIsLandscape(UIDevice.currentDevice().orientation)
}