make .btn:focused styles apply to .btn.active:focused

[cvrebert]

Proposed fix for #12154.
/to @mdo for review.

[mdo]

What about :active?

[cvrebert]

Hmm, I wasn't sure that :active:focus was a thing, but apparently it is. I'll update this accordingly.

[cvrebert]

@mdo Updated. Test bench: http://jsfiddle.net/n6Bc2/5/
Would be more elegant if we didn't have to contend with the outline: 0; on .btn:active, .btn.active.

[mdo]

Hmm, only other thing to think about is how this overlaps with dropdown buttons, button groups, and input groups. Currently we override the focus and outlines for those since the JS manually reapplies focus to the buttons when toggling content. The focus ring is seen as unattractive in those situations.