Accommodate new lighting and color management

[SeanCurtis-TRI]

We've upgraded three.js from 0.149 to 0.156. This led to changes in how colors are managed and lighting calculations are performed. To account for those changes and give a qualitatively similar experience with the default values, we've made the following changes:

• Light intensity controls now have units (candela) displayed.
• The old (non physical) light intensity values have been boosted to be more or less equivalent to what they were before (see documentation in code).
• The color space of the background images has been corrected (to sRGB).

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  This change is 

[kielnino]

@jwnimmer-tri I would like to point out that this also affects the companion repositories.