v4: hard-coded border-width properties

Created by: davidjb

Looking through v4-dev, the $border-width variable is available, but only sporadically used (list-groups, navbar, hr, etc). There's a variety of different places 1px borders are hard-coded (dropdowns, nav, model, images, tables) and then components such as nav use a negative margin for positioning over the border.

Would a PR that changes these 1px references over the $border-width be acceptable, or are there particular components that require precisely 1px? Alternatively, should components each have their own configurable borders and just default to $border-width (ala #17939)?