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;
```

app/assets/images/folder-black.svg

@@ -0,0 +1,13 @@
+<?xml version="1.0" encoding="utf-8"?>
+<svg version="1.1" id="blue_outline" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" x="0px"
+	 y="0px" viewBox="0 0 26 20" style="enable-background:new 0 0 26 20;" xml:space="preserve">
+<style type="text/css">
+	.st0{fill:#DEE0E2;}
+	.st1{fill:#0B0C0C;}
+</style>
+<g>
+	<polygon class="st0" points="1,19.1 1,1 9.8,1 11.3,5 25,5 25,19.1 	"/>
+	<path class="st1" d="M9.2,1.9l1,2.8l0.5,1.2H12h12.1v12.2H1.9v-11V5.2V1.9H9.2 M10.5,0h-0.2H0v5.2v1.9V20h26V4H12L10.5,0L10.5,0z"
+		/>
+</g>
+</svg>