Input group sizes: Properly position .form-control-feedback

[hnrch02]

Ref #12868 (comment)
Ref 7733f24

Before: http://jsbin.com/hubiju/1
After: http://jsbin.com/hubiju/2

The reason I didn't convert the selector into something like this

.input-lg,
.input-group-lg {
  + .form-control-feedback { ... }
}

is that it would be mixed into here.

/cc @mdo

[mdo]

Looks good to me I think.