Sky gradients

• New Skrollr-blending `linear-gradient`s on sky-phase's `.back.fill`.
	» Up-front call to `.linear-gradient()` mixin (see below) with the initial `@sky-eve-*-as-rgb` colors.
	» `-skrollr-dry-animation() {}` call with:
		◦ a `_sky_phase_start` keyframe that provides a plain-W3C-CSS `linear-gradient` with the same “initial” `@sky-eve-*-as-rgb` stops
		◦ and a `_sky_phase_center` keyframe with a `linear-gradient` with `@sky-night-*-as-rgb` stops (may change this to `_sky_phase_end` later, depending on the final opacity of the ambitions-phase's back)
	» I like the blending between `-as-rgb` colors better than `-as-hsl` colors.  The former is duller, and looks more like a real sunset, where the latter is just a bit too unnaturally vibrant.
	• `background-color` fallbacks set to the `-mid` color.
	• Sky's text `color` changed to opaque white, and given a soft (3px) 50% opaque black soft shadow, since that's a trick to make text contrast a little better.
• Added a `.linear-gradient()` mixin because Bootstap's `.vertical()` wasn't cutting it.
	» Bootstrap's doesn't support 3 colors… which I now see it has `.vertical-three-colors()` for… but it also doesn't support 4+ colors or any direction.
	» This simpler mixin mostly just passes stuff through, so any `@dir` and `@stops` will be taken as written.
	• `#ambitions-phase` and #title-phase` both switched to use `.linear-gradient` instead of Bootstrap's `#gradient > .vertical()`
Tweak: Blue-tinted `@sky-night-mid` by 40°

• `spin()`ed (hue-shifted) `@sky-night-mid` from its `@sky-night-mid-pure` source -40° to make it bluer and fit better between the other two.
	» Instead of `multiply()`-ing it.  It was dark enough already; multiplying made it too dark.