BS3: `.btn-group`s for your `.btn-group`s

This fixes #9242—the inability to nest button groups in vertical button groups.

As a side note for @ggam, if we had added #9407 this wouldn't really be possible because we'd have to reset the original `border-radius` values that are required by the vertical group. The goal with how we wrote these is that we don't ever have to reset an original value, only zero out what needs zero-ing out. Hope that helps!