Fix: Nesting `& when not (…) and (…) {`

• Nesting the two conditions of `.generateStarfieldRules()`' `& when not (…) and (…) {` separately to achieve the same intended effect.
	» LESS's implementation appears to be buggy— when passed an @initialOpacity of `0.0` and an @endOpacity of `0.5` it incorrectly compiles out the contents.
Vertical Centering

• New `.vert-middle-block` accomplishes vertical-centering by setting its element to display as `table` with 100% width & height, and each of its children to be mandatorily displayed as `table-cell` and to have a `vertical-align: middle`.
	» The `table-cell` is `!important` because it won't work otherwise.  If this conflicts with other rules, we want to know about it so we can added ad intermediate “inner” element, or add additional edge-case rules here.
	• A `.vert-block()` mixin actually supplies all the rules, taking only a `@verticalAlign` arg.
	• Also generating CSS for `.vert-top-block` and `.vert-bottom-block` variants, because why not.
• Wrapped each `.monologue` div in index.html with `.vert-middle-block` to vertically center each.
	» `.monologue`s shouldn't care that they're positioned as `table-cell`s now.
	» title-phase's `.vert-middle-block` got an `.inner` div because we want (for now) the children to be block elements, not side-by-side table cells.
.vh-75 classes

• Added `.vh-75` classes under `.screen-`/`.base-`/`.line-height-rel` with a multiplier/unit of 75.
• Changed the 2nd & 3rd `.monologue`s in sky-phase from vh-50 to vh-75.
Reorg: main.less headers

• Renamed the “Bootstrap default overrides” header to “Text Styling” and moved the `.monologue` rule under it.
• Renamed “full-screen-height adjustments, rules” to “Layout”.
• Capitalized the rest of them.
Revised constants: _appear/_open/_close/_disappear

• New `*_appear` and `*_open` constants discern the difference between starting to appear on the screen (element-top at viewport-bottom; formerly `*_start` for most), and being fully present on the screen (element-top at viewport-top).
	» Sky-phase doesn't get a `sky_phase_appear` because it's the first section— so that'll never happen.
• New `*_close` and `*_disappear` constants discern the difference between covering the screen but about to scroll off (element-bottom at viewport-bottom), and having just disappeared off the top of the screen (element-bottom at viewport-top).
• Updated `#sky-phase`'s `sky-fill-gradient` animation to run to `_sky_phase_close`, so we get to see the full transition.
• Updated existing usages of `*_start` to `*_open` and `*_end` to `*_disappear`.