- Drops the absolute positioning of the icons within the left/right controls. We have to keep the controls themselves positioned though since we're overlapping HTML elements here.

- No more position, left, right, or margins involved; just some justify-content and align-items.

- Add some comments for explaining which flex property-value pair does what.

- Remove the unapplied font and line-height stuff now that we're all SVGs and flexbox here.

This revamps the indicators to use flexbox instead of inline-block for added flexbility (hah). Indicators now automatically scale based on the number of elements present, and max out at the `$carousel-indicator-width` instead of always being that wide.

- Require inner element for the icon for improved customization (e.g., drop the element to replace it with your own icon font or SVG)
- Tighten up padding
- Better comments

fixes #18115: move forward with one naming scheme for inversed color schemes, renaming .navbar-dark to .navbar-inverse (matching .card-inverse in the process and restoring v3's inverted class name)

update justified nav to use justified navbar; remove safari bug warning given flexbox implementation

move all nav link and collapse and container behaviors to the toggleable class generation area; this way the .navbar-toggleable-* goes on .navbar, easily affecting everything that needs updating for each breakpoint's use