Add new Horizontal List Group class.

Created by: sts-ryan-holton

Hi 👋

🚀 Preview: https://deploy-preview-27683--twbs-bootstrap4.netlify.com/docs/4.2/components/list-group/#horizontal-list-group

Back in September I opened a new feature request about adding a new horizontal list group class to the existing list group (see: https://github.com/twbs/bootstrap/issues/27291) as I felt that although this could be done with pure utility classes it was difficult to get the border radius to match up.

Here, I am opening a new PR containing my first attempt/implementation at adding a few new classes in addition to the existing .list-group class which shouldn't contain any breaking change, but rather extend the current classes.

Here's a breakdown of the initial changes that this PR contains (open to additional contributors):

• Add new .list-group-horizontal modifier class.
• Add new .list-group-horizontal-responsive class for dealing with responsive horizontal list groups (this also works with the existing -sm|-md|-lg|-xl variants)
• Add support for the .list-group-flush class to horizontal list group.
• Update List Group docs with the new Horizontal List Group and Responsive variant.
• Test with existing modifier classes such as: .active, .disabled and with <a> elements.
• Add new border-radius mixins to support the new classes.
• Utilise as many existing variables and primary extend only.