Fixes #26897

ref: https://github.com/twbs/bootstrap/pull/23881#issuecomment-337789596

* add 'lighter' and 'bolder' font weight classes

these are 2 special values for font weight, that will give their content a font-weight value of 100 more or less than their inherited font-weight.

probably doesn't fully fulfill this issue, https://github.com/twbs/bootstrap/issues/23969, but it's a start

* Update .stylelintrc

* add 'lighter' and 'bolder' variables per @mdo 's request.

Nest `.modal` styling directly under `.modal-open`
to avoid issues when bootstrap is nested.

* `svg:not(:root)` specificity is very high (https://github.com/necolas/normalize.css/issues/718)
* Bootstrap do not support SVG documents (See #26878)

Fixes #26227

Updates the properties and values for which we have existing variables to match our tooltips. Come 4.2, we'll create new tooltips for these.

* Redo input height on .form-control

- Use the already present -height variables on .form-control
- Consolidate the select size and multiple overrides into the .form-control base class instead of sm/lg modifiers
- Remove the Sass extends from input groups since it picks up too many selectors

* Prevent height on textareas

* Fix focus box shadow of custom ranges

* Put `:focus` selector right after `.custom-range`
* Remove unnecessary `outline: none`

* Fix box-shadow issues in IE/Edge

* Better align

* Fix thumb vertical positions in IE/Edge

* Fix incorrect formula of thumb vertical positions in Webkit

* Fix an incorrect comment

* Fix incorrect box-shadow-width

Now `.custom-control` will have height equal to `$font-size-base`
Fix custom checkbox and radio top position (we can't subtract rem from unit)
`$line-height-base - $custom-control-indicator-size` is not valid
Because `$line-height-base` is a unitless variable and `$custom-control-indicator-size`
is a unit (rem) variable

- Fixes #25656 where custom selects had the wrong focus shadow
- Fixes #26668 where custom selects had no inset shadow (also closes #26572 which had the wrong form classes on the first example anyway)

Feature request #26681

By applying the transition: CSS property only to classes that are
active during sliding, we avoid an unnecessary, non-zero-time
animation that although mostly invisible, does interfere with e.g.
z-index based parallax