Add API key component

This commit adds a component for showing an API key. Usage:

```jinja
{{ from 'components/api-key.html' import api_key }}
{{ api_key('e1b0751388f3cd0fc9982c701acdb3c2') }}
```

Depending on the user’s browser, it works in three different ways.

No Javascript
---
The API key is shown on the page.

Older browsers with Javascript
---
The API key is hidden, and users can click a button to reveal it.

Newer browsers that support copying to clipboard without Flash
---
As above, but when the key is shown there is a button which copies it to the
clipboard. This is acheived by using
[this polyfill](https://www.npmjs.com/package/query-command-supported)
to reliably detect browser support for the ‘copy’ command.

The styling of the component is a bit different to the initial sketch. I think
a grey button works better than green. Green feels like it’s going to take you
somewhere else.

app/assets/stylesheets/components/api-key.scss

@@ -0,0 +1,17 @@
+.api-key {
+
+  &-key {
+    font-family: monospace;
+    display: block;
+    margin-bottom: 10px;
+  }
+
+  &-button-show {
+    @include button($grey-3);
+  }
+
+  &-button-copy {
+    @include button($grey-3);
+  }
+
+}