Use new `.breadcrumb-item` class instead of child selectors and `li` tag selectors,
so as to no longer require the usage of `<ol>`-based markup.
Rename variables to follow naming conventions:
* $breadcrumb-padding-vertical => $breadcrumb-padding-y
* $breadcrumb-padding-horizontal => $breadcrumb-padding-x
Introduce new variable: $breadcrumb-item-padding
[skip sauce]
We have it set to `1` right now just to avoid compilation errors, but
it’s been replaced everywhere with our new spacer classes anywho. We’ll
likely want to remap that var to custom component vars though (e.g.,
`$pagination-margin` instead of `$spacer-y`).
