PN-7736 Synchronize skin detection between JSP and JS

Checking classes at the body level:
- if only one of these 2 classes are on the body (wf2-skin-nx and wf2-skin-nxt), it means it's intentional or set by the JSP page tag: load the corresponding skin.
- if none of these 2 classes are present or both are present, then use the touch detection mechanism to load the right skin. (in that case, we know that the class was NOT set by the JSP tag).
This seems very specific to nx vs nxt. I know it's an interim solution, but we could probably make it a little more future-proof in the off case that we don't get our new solution in place soon. I'm thinking matching all of our potential skin classes a la `/wf2-skin-nx([a-z]+)?/g`