fixes border-width issues when customizing font-size by using 1px instead of .0625rem as default -width

Added usage of $border-width variable in all _list-group.scss references

In Bootstrap 3, the menu collapsed on the -sm- breakpoint, I believe the equivalent of this breakpoint in Bootstrap 4 is actually the -md- breakpoint. The navbar currently has no option to collapse at the -md- breakpoint and I'd like to add this.

Fix #17166

Missed one variable :(

Signed-off-by: O'meid <public@omeid.me>

If we want to support namespaced import:

```scss
.twbs {
  @import 'bootstrap';
}
```

We cannot use `selector &`, see sass/sass#1817.

`fieldset[disabled] &` is not needed as we no longer support IE8.

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Perhaps I'm a little slow & need to have things spelled out :)
https://github.com/twbs/bootstrap/issues/17301#issuecomment-134469886

inline the one remaining usage into .hidden-print's definition
rm now-empty scss/_utilities-responsive.scss file

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Properties should be ordered border-top, border-bottom

Fix #17288 — Sorry about that!

Fix #17052

- Add explicit -grid-classes variable and set to true by default
- Wrap .row and .col generation in the boolean var

!default to all variables.

Fix #17144

Fix #17117