Fixed-position sky back (finally)

This is necessary since iOS devices don't react to fixed-position backgrounds in the same way, and fixed backgrounds limit us to just `background`s, excluding positioning via `absolute` `top`, `transform` `translateY`, `canvas`/`svg`, or others.

I hadn't been able to get this working yet because Skrollr was mis-calculating its keyframes and stopping the transition early.  Skrollr's dynamic constants let me base starfield animations off of the viewport position of its containing phase.

• All of sky-phase's .back divs (fill and starfields) wrapped in a new .back.fill div.
	• New `&.fixed` (under .phase's .back) CSS sets the `position` to be `fixed`.
		» Selector evaluates to `.phase > .back.fixed, .phase > .back .back.fixed`, and the same for each `#…-phase` id.
• Removed `fixed` from each of the starfield's `background`s.
• New Skrollr constants in the form of `(sky|ambitions|title)_phase_(start|center|end)` to supply document positions of each phase to the `@-skrollr-keyframes` keyframes.
	• New `get*PhaseScroll*()` functions to get the scroll center of the start, center, and end of each phase, for use by the dynamic constants.
• Moved `> .fill` CSS to `&.fill` under each phase's `> .back { &, & .back {` in order to have it keep working while excluding any non-phase back (any element with .back that isn't under a .back directly under the phase).
Fill specificity
Fix: Added z-indexes to each phase, for iOS rendering

• Added `z-index`es of 100, 200, and 300 to the sky-phase, ambitions-phase, and title-phase divs.
	» For some unknown reason, iOS renders the fixed-position stars in the sky-phase over the content of the ambitions-phase.  This forces the intended order.