Use autolayout constraints to set size of custom bar button item (#4732)
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