Added “opacity != 1.0” guards to `starfield*op` animations

• Added `@endOpacity` vars to each starfield with the values formerly in the 2nd `starfield*op` keyframe, and set each of the second keyframes to use `@endOpacity`.
	» This is so that we can check the value beforehand.
• Added a `& when { … }` if-style guard around the `.opacity()` and `.-skrollr-dry-animation()` calls, with conditional logic to skip it if both `@initialOpacity` and `@endOpacity` are `1.0`.
	» If we're both starting at full opacity and ending at it, there's really no reason to have the opacity rule or to have Skrollr set up an animation.
• Minor: Renamed `@initial-opacity` vars to `@initialOpacity` for consistency.
Minor: Enabled LESS strictMath & strictUnits

• Enabled the `strictMath` option, which only performs LESS math when a set of extra parentheses are around the operation(s).
	» I like this because it makes it clear that the early (compile-time) calculation is intentional, rather than wanting a later (browser-time) calculation (e.g. with `calc(…)`).
	• Updated existing math here & there to have the extra parenthesis-wrapping.  (Honestly, more of my not-yet-finished-and-checked-in code uses math than what's currently committed.)
• Enabled the `strictUnits` option, which illogical math like `3px * 5px` throws errors (since px*px is px², and CSS doesn't have units for areas).
	» I wasn't in violation of this one, but seems to be a nice safety guard for late-night coding sessions.  LESS HARD MODE ACTIVATE!!@!!!!!!!11#@^
Caching phase elements and inline Skrollr-constant functions

• New `cacheIdElement()` function finds the element with the given ID (throwing an error if not found, cause that's kinda a big deal), converts the id to a js-friendly camel-cased name prefixed with `$` and suffixed with `Element`, and sets a var on `window` with the $-name to the element so all other scripts can access it (as a global, essentially).
	• Three calls cache references to the `sky-phase`, `ambitions-phase`, and `title-phase` elements, and safely (AFAIK) in JS running in-line at the bottom of the page, earlier than `document.ready`.
• Made each of the `get*PhaseScroll*()` functions use the new `$*PhaseElement` global, anonymous, in a single line after each Skrollr dynamic constant declaration.
	» Ultimately, this should perform faster at scroll-time without any load-time stipulations.