This formats the text users enter on the card donation screen. For
example, "4242424242424242" becomes "4242 4242 4242 4242".
It uses [the `textField(_:shouldChangeCharactersIn:replacementString:)`
method of `UITextFieldDelegate`][0] to accomplish this, and implements a
helper that can handle all the cases (inserting, deleting, replacing,
pasting, and so on).
Tested this in an iPhone 14 simulator and on a physical iOS 12 device.
[0]: https://developer.apple.com/documentation/uikit/uitextfielddelegate/1619599-textfield
