Support High Voltage when generating page classes

High Voltage gem: https://github.com/thoughtbot/high_voltage

Using the current implementation, all High Voltage pages will be generated with the `.high_voltage_pages.show` page class. This doesn't allow us to distinguish one static page from another in our CSS.

With this fix, an exception is implemented for High Voltage controllers, and the actual page that is being displayed will be used to generate the page class.

To prevent a dependency on the High Voltage gem, I have provided a stub.

Comments welcome :)