leftovers from renaming from .open to .shown

Created by: wojtask9

There were some changes to opened/expanded/shown state of widgets This commit https://github.com/twbs/bootstrap/commit/9d129a43d6c24d9e60cc8fc7cfbddaaa4c586fdf renamed dropdown from ".open" to ".active" class

Another commit commit https://github.com/twbs/bootstrap/commit/bf39bb3ac3d2aef4687b3cd4762015d5f218e2bc renamed from ".active" to ".show".

But _navbar.scss weren't updated (should be .open -> .show) See https://github.com/twbs/bootstrap/blob/v4-dev/scss/_navbar.scss#L217 for example

Another file with wrong class: https://github.com/twbs/bootstrap/blob/v4-dev/scss/_button-group.scss#L83

This ".open" classes are leftovers and currently should be renamed to '.show'