Add optional `$color` argument to `button-variant()` and
`button-outline-variant()` for additional flexibility.

* Scope .form-check-input layout to the .form-check parent

* Remove margin-bottom from list-group-items in case they are label elements

* document .form-check in input groups and list groups, and cleanup the no-label examples in the main .form-check docs

* Remove outline from select box in FF

* Use $form-select-color

- Rename and move the variable to variables file
- Move code to the grid file
- Use the mixin to generate our own classes
- Wrap in a grid classes enabled conditional
- Document the mixin

Trying to find a new way to do responsive card decks while not locking ourselves into the cards themselves. My thinking here is we can easily control the column (.col) width by the parent, but I don't know how many we need (have 0-5 now) across each breakpoint. This works for cards so far, and I think could get us equal height, too.

Also, add a warning in _root.scss so that we don't miss it again.

- `.carousel-caption` doesn't need a z-index. There are no non-static element in its containing block (`.carousel-item`)
- The `z-index` of `.carousel-indicators` is way higher than it needs to be (`15`), the highest sibling `z-index` is `1`, so it can be set to `2` to be the top layer.

- Add example to the new form control docs
- Reset the [list] selector in Reboot to hide the random dropdown arrow in Chrome

* set `min-height` for `.form-control` textarea
  * prevent user from shrink textarea to minimum height