Grow button width to the parent button group

Created by: ysds

Even if you specify a width to a button group, the width of the children buttons don't change.

e.g. set width 300px to a button group:

This PR change that the width of each button will grow according to the width of the button group.

Also this fixes #27702 (closed)

To reproduce, please change the "Dropdown" text in:

• Before: https://deploy-preview-25395--twbs-bootstrap4.netlify.com/docs/4.1/components/button-group/#vertical-variation
• After: https://deploy-preview-27717--twbs-bootstrap4.netlify.com/docs/4.1/components/button-group/#vertical-variation