- no real need for everything to be 100% shared
- padding looked and felt too large for inputs but not for buttons
- tying forms and buttons seems fine, but throwing in pagination feels wrong

- Fixes #13267 somewhat.
- Instead of a single, block-level class, let's use a combination of existing elements and classes.
- For block-level help text, use p.text-muted.
- For inline-level help text, use span.text-muted or small.text-muted.

Because we use `position: absolute` on our inputs, when there’s no
label the `.checkbox` ends up having no computed height. To avoid
rendering errors when there’s no label text, we reset the `position` to
`static` for normal rendering.

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`).

replaced media-xs => media-breakpoint-down(xs)
replaced media-sm => media-breakpoint-up(sm)
replaced media-sm-max => media-breakpoint-only(sm)
replaced media-md => media-breakpoint-up(md)
replaced media-md-max => media-breakpoint-only(md)
replaced media-lg => media-breakpoint-up(lg)

Breakpoints and container widths are now defined as maps.

:disabled is supported in IE9+ and all modern browsers

Mozilla fixed the bug circa Firefox 33-35:
    https://bugzilla.mozilla.org/show_bug.cgi?id=763671
Firefox 34 is the current version.
Firefox 35 should be current by the time Bootstrap v4 is released.