Fixes #21956

The actual available size of the navbar toggler icon is 30px, but the
SVG uses a 32 unit grid. This commit uses a 30 unit grid and updates
icon accordingly.

* Remove unused scss variables

* Remove unnecessary space

* Remove $zindex-navbar from docs

* Re-add $progress-box-shadow

Remove .navbar-divider; it's undocumented and wasn't updated with the flexbox navbar updates, so it's kinda bunk right now

- had to rearrange vars to re-use heading line-height
- align values while we're at it

- <progress> element didn't allow animation, labels overlaid, multiple bars, etc.

- Revamps CSS to use something more similar to v3's implementation

- Ditches variant mixin for `bg-` utils

- Rebuilds docs to match, including adding a new Height section for customizing that.

Only potential remaining todo is adding `.sr-only` instances to within the bar. Unsure if that's necessary.

- Overhauls the states, including the link/button variants, for list groups to better match how we handle .btn states.
- Moved the .list-group-item-action styles before the .list-group-item so that we don't have to do as much overriding.
- Removed the plain-hover-focus mixins from the disabled and active states since they're unnecessary.
- Added support for :active states on the .list-group-item-action variant (for the current click state).
- Removed the heading and text classes and variables since we can accomplish the same thing with utilities.
- Added support for :disabled on the .list-group-item-action as well since we can use those with button elements.
- Rearranged docs to reflect all the above changes.
- Reformatted some Sass variables.

- No need to set hover and focus on disabled or active elements. Disabled has no interactivity and active will inherit the focus.
- Also removes two now unused vars.

also be sure to apply it for the active state when shadows are enabled

Use pixels instead of rem units for focus box-shadow on custom controls so that IE11, Edge, and Win FF screw it up less

Turns out we did have the correct height calculation with our custom selects. The problem was we lacked a shared line-height with our buttons and inputs.

This restores the previous `calc` math and adds a line-height that reuses the input line-height.

* Add color variables for white and black

Closes #21395.

this assumes no margin on the .card to start (which will follow in a later commit)

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)

* remove the $enable-flex variable option

* remove bootstrap-flex.css dist file and it's grunt task

* remove the separate flex css file for docs; it's all the same now

* remove flexbox docs (porting some to the main grid docs in next commit)

* clean up few grid docs bits to simplify copy, start to mention flexbox

* port relevant flexbox-grid.md content to grid.md

- clean up mixins
- update how it works section
- bring over sizing and alignment sections

* remove the $enable-flex from the options.md page

* update lead paragraph to mention flexbox

* update migration to mention loss of ie9 support

* remove mention of flexbox dist file

* clarify IE support

* making a note

* remove flexbox variant mentions from component docs

- updates docs for media object, navs, list group, and cards to consolidate docs
- no more need to callout flexbox variants since it's now the default

* remove $enable-flex if/else from sass files

* remove flex dist files

* update scss lint property order to account for flex properties

* linting

* change to numberless classes for autosizing, wrap in highlighting div

* bump gruntfile and postcss to ie10

* redo intro sections

* rearrange

* phew, redo hella grid docs

- rearrange all the things
- consolidate some bits

* remove reference to flexbox mode

* more border action for demo

* Make some changes to the .card's in .card-deck's to ensure footers align to the bottom

This pulls in some changes from #18462 to include a set of generic color variables. It doesn't include a Sass map for generating the theme colors yet, since we can't easily do that for all components, but does give a few more color choices to folks and an easier way to customize.

* Add variables for transitions

* Add variables for transitions