sass: remove all uses of `/` as SASS division (redo) (#1077)

Context: #1074, #1076. I think the problem is likely with `@use "sass:math";`; the stock pages image doesn't contain an up-to-date enough version of SASS. I've instead replaced just that instance with a runtime `calc()` operation, which *should* get optimized away by the compiler (see: [SASS docs](https://sass-lang.com/documentation/breaking-changes/slash-div#transition-period)).

---

Original PR body:

> @pdmosses noticed that we have deprecation warnings on some of our SASS code. After testing locally, all of them have to do with using / as division in SASS, which is [deprecated](https://sass-lang.com/documentation/breaking-changes/slash-div) (since there's some lexical ambiguity).
> 
> SASS has a nifty [migrator tool](https://github.com/sass/migrator). I used the migrator piecewise on each deprecation warning (since the global usage fails on some liquid code). Upon manual inspection, I think there are no false positives. Running bundle exec jekyll serve after a fresh install and bundle update no longer emits any warnings.