Button-as-module to switch scenes

[jonthegeek]

As a {scenes} user, there are cases where I want to make it easy for the user to switch between apps (by switching a query parameter on the fly), so that I can code the modes as completely separate apps.

Simple module that updates the URL to switch query parameters.

Ideally this should come after we can wrap at the ui + server level to make it really two separate apps.