Prototype an idea for nested label inputs.

Add a new label template and modify the existing templates to make it easy (and the default) to have radio/checkboxes embedded in their labels. Because bootstrap is pretty common this layout gets requested pretty often for checkbox/radio inputs.

This still needs tests written/updated but I wanted to get some early feedback on the idea and approach. This is intended to be an alternative to #4216
The class name feels weird since it's the input that's nested not the label.