Split sky shader's twilight into dawn and dusk

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Addressing #1538 (closed).

Instead of having the same color profiles for the sky's color, light, and halo, we now have the ability to define different values for dawn and dusk.

The dusk values were unchanged and the dawn values are similar, but with less red. These should probably be tweaked a bit more.

Below are a couple of screenshots (cloudy and clear sky) of the old dawn (now only dusk) and new dawn:

mentioned in merge request !3373 (closed)

Mentioned in today's weekly meeting and the general consensus was to merge.

approved this merge request

started a merge train

mentioned in commit 8e0295ff

merged