This fixes an issue where the frame for the custom view can be set to the incorrect y-offset upon setting the custom frame.

In iOS 11, this behavior changed, as UIBarButtonItem went from being using springs-and-struts for sizing, to using a UIStackView, and thus using Autolayout.

This lead to the superview of having a frame of (0, 22, 0, 0) at the first layout pass.

By moving to using NSLayoutConstaints, we can now properly size our custom view.

See also: https://gist.github.com/niw/569b49648f
See also: https://stackoverflow.com/questions/10988918/change-width-of-a-uibarbuttonitem-in-a-uinavigationbar