Add larger version of black folder icon
Add larger version of black folder icon

This is for use in the folder permissions UI. It’s designed to be sized
at the same width as a GOV.UK style checkbox. The CSS to render it is
something like:

```css
background-image: file-url('folder-black.svg');
background-repeat: no-repeat;
background-size: 39px auto;
background-position: 0px 4px;
```
