Update the sky box to use the correct orientation.

The previous star images had at least two problems:

* The inclusion of "border pixels" (for STK's benefit) that messed up WebGL cube mapping

* The wrong coordinate system, called "Galactic" coordinates (not a joke) where the Milky Way is the `XY` plane instead of the Earth's equator.

This pull request also adds a `skyBoxBaseUrl` parameter to the widget, for easily swapping out different sets of SkyBox images.

My hope is this pull will also address #426.