Fluid Layouts with Auto Layout, Size Classes, Spacer Views, and Constraint Priorities

I came across a problem the other day with the "compact x any" size class in iOS 8 & Xcode 6. I was creating a layout for the size class which is supposed to cover the iPhone 4S 3.5-inch screen all the way up to the iPhone 6 4.7-inch screen. However, this became difficult to do well because what looked good on a small screen (like the 3.5-inch) did not look good on the large screen (4.7-inch...