Add an extension for accessing the AppDelegate from ViewControllers

This adds an extension to grab the AppDelegate from any ViewController. I've also added a property (`AppDelegate.signedInUser`) that represents the user that's currently logged into the app. @snekkalapudi when the user signs up or signs in, you can set the field from your view controller like so:

```swift
class AuthViewController: UIViewController {
  func signin(user: User) {
    appDelegate.signedInUser = user
  }
}
```