If we want to support namespaced import:
```scss
.twbs {
@import 'bootstrap';
}
```
We cannot use `selector &`, see sass/sass#1817.
`fieldset[disabled] &` is not needed as we no longer support IE8.
- 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
