Correctly store data-url and correctly select for it

This also removes any lingering references to ```subPageUrlKey``` from the code.

Fixes gh-1383
This value is created from the passed in absUrl, not the data attribute -- i.e. this is the page we are navigating TO. So this is the first check to see if it is in the DOM.

In my form submission scenario, absUrl contains the serialized form parameters, which includes the encoded characters.