[NT-796] Fix sheet overlay on iOS 13

[justinswart]

📲 What

• Fixes the presentation of the SheetOverlayTransitionAnimator on iOS 13 so that the view extends beyond the safe area.
• Uses default modal presentation on iOS 13 for the login and add new card modals during checkout.

🤔 Why

This was a regression noticed after updating to iOS 13. The default modal presentation style is sufficient in 2 of 3 places that we use the SheetOverlayViewController, however for the presentation of the PledgeShippingLocationViewController we still need it for the custom vertical offset behaviour.

🛠 How

• Fixed the frame sizes in SheetOverlayTransitionAnimator.
• Added #available checks for iOS 13 to default to the built-in modal for that OS version.

👀 See

iOS 12	iOS 13	iOS 13 login

✅ Acceptance criteria

iPhone running iOS 12.4:
iPad running iOS 12.4 or iOS 13.x.

• Tapping continue on pledge view when logged out presents login modal using SheetOverlayViewController.
• Tapping add new card on pledge view presents AddNewCardViewController using SheetOverlayViewController.
• Selecting a shipping method presents PledgeShippingLocationViewController using SheetOverlayViewController.

iPhone running iOS 13.x:

• Tapping continue on pledge view when logged out presents login modal using the standard modal presentation style.
• Tapping add new card on pledge view presents AddNewCardViewController the standard modal presentation style.
• Selecting a shipping method presents PledgeShippingLocationViewController using SheetOverlayViewController.