v4: Inverse table variants

Add disabled styling for .custom-select

Added explicit commentary to .card-* classes

Update Normalize to v4.0.0

- Change some things around, add some copy
- Include table variant styles for the inverse tables thanks to bg and color utils

Add GitHub issue template

Update overview.md

Jank card borders were too janky

Add $card-img-overlay-padding variable

Extract $carousel-icon-width variable

- Drops the experiment I had going for `box-shadow`-powered borders
- Reinstates regular `border` using existing variables

Fixes  #19097 and #19143. Nullifies #19828.

Use alphabetic order for the Components doc menu

[skip sauce]

endtag for the highlight

add the `@include media-breakpoint-only()` and `@include media-breakpoint-between` mixins

Suppress IE/Edge additional white/blue colors for focused <select>

Modal: Remove IE8-specific window.innerWidth workaround

[skip validator]

Add Wall of Browser Bugs entries for WebKit media query bugs
[skip sauce]

These bugs are factors in our decisions regarding which unit to use in our media queries.
Refs #17403
[skip sauce]

remove `/` in reference url (otherwise 404s)

.navbar-divider width: 1px=>$border-width