Add some useful "feature" classes to the <html> el

Allows CSS to know what features are enabled and adjust the styles to
compensate.
Disable scores for the session with the query string

On the root page, or a scenario page ("modern" or "house"), include the
"scores" parameter in the URL to set whether to display the high score
podium and high score list.

To disable scores, any of the following will work:

  * scores=off
  * scores=no
  * scores=false
  * scores=hide

Any other value will turn scores back on.

For example:

  * http://beta.etflex.et-model.com/?scores=off
  * http://beta.etflex.et-model.com/?scores=on
  * http://beta.etflex.et-model.com/scenes/1/with/348179?scores=hide

Closes #418.