Button groups no longer apply their own border or border radius, instead

using those of the buttons they contain. This avoids some

inconsistencies between border radiuses (where the inner was a bit too

small, if you zoom in), and keeps button borders consistent with that of

the inner buttons (noticeable when a button group consists only of

primary or danger buttons).
A very thin (half a pixel) border is also now applied between buttons,

making it easier to see where one button ends and another begins.

Tested this in Storybook, and with the in-progress MenuButton.