Combined starfield & image divs

• Combined .starfield divs and the .image divs into a single div.
• Changed .image CSS selectors under starfields to `&.image, & .image` in order to apply to either a contained .image (as it was) or .image on the starfield itself (as it is now).
	• Using `&:extend(.starfield all)` to properly inherit all `.starfield` and `.starfield … .image` CSS w/o including it twice.
• Changed all starfield animation keyframes to start at `_sky_phase_start` and end at `_sky_phase_end`.
	• Changed opacity easing types to `linear` or the new `linearStartToMid` and `linearMidToEnd` in order to preserve starfields appearing halfway through or becoming fully opaque by halfway, without affecting the movement animation.
		» linearMidToEnd could be wasteful here, since there's no reason to animate the movement when it's not visible, but it probably won't be a big deal so I'll optimize it later if need be.
	• Added `linearStartToMid` and `linearMidToEnd` easing functions to the skrollr config, the former which returns 0 at or under halfway, and lerps the other half, and the latter which lerps the first half and returns 1.0 for the other half.
Converted starfield declarations into a reused mixin

• Created a new .generateStarfieldRules() mixin which takes:
	◦ @uniqueName, used for -skrollr-animation-name naming
	◦ @intialOpacity & @endOpacity
	◦ @initialBGOffset & @endBGOffset, any-unit Y offset value
	◦ @opacityEasing & @bgEasing, the Skrollr easing types
	» Specifies all the needed rules except the `background-image` itself.
	» (Still) extends `.starfield` so we get anything we put in there too.
	• Called 4 times from within `#starfield1`-`4` blocks, which also specify the `background-image`.
Scroll offsets made base-relative.

• New `@starfieldScrollOffsetBase` provides a base speed (including direction).
• `.generateStarfieldRules()` now takes `@initialScrollOffsetFactor` and  `@endScrollOffsetFactor`, which are multiplied against the base before being set.
	• Calls to `.generateStarfieldRules()` updated to pass just the factors: for the initial factors, `0.0`; and for the end factors, the constant part of the former multiplication math.