add "visually hidden" mixin/helper

add a class or Sass mixin to hide something in a manner that is still accessible to screen readers

[html5bp](https://github.com/h5bp/html5-boilerplate/blob/master/dist/css/main.css#L113-L133) uses:

```css
.sr-only {
border: 0;
clip: rect(0, 0, 0, 0);
height: 1px;
margin: -1px;
overflow: hidden;
padding: 0;
position: absolute;
white-space: nowrap;
width: 1px;
}
```

Start by reviewing the linked html5bp CSS implementation and locating the project's Sass entry point. Add a class or Sass mixin that visually hides content while keeping it accessible to screen readers; done means the requested helper is available in the frontend styles.

Written by the indexing model from the issue text.