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/javascripts/apiKey.js

@@ -0,0 +1,58 @@
+(function(Modules) {
+  "use strict";
+
+  Modules.ApiKey = function() {
+
+    const states = {
+      'initial': `
+        <input type='button' class='api-key-button-show' value='Show API key' />
+      `,
+      'keyVisibleBasic': key => `
+        <span class="api-key-key">${key}</span>
+      `,
+      'keyVisible': key => `
+        <span class="api-key-key">${key}</span>
+        <input type='button' class='api-key-button-copy' value='Copy API key to clipboard' />
+      `,
+      'keyCopied': `
+        <span class="api-key-key">Copied to clipboard</span>
+        <input type='button' class='api-key-button-show' value='Show API key' />
+      `
+    };
+
+    this.copyKey = function(keyElement, callback) {
+      var selection = window.getSelection ? window.getSelection() : document.selection,
+          range = document.createRange();
+      selection.removeAllRanges();
+      range.selectNodeContents(keyElement);
+      selection.addRange(range);
+      document.execCommand('copy');
+      selection.removeAllRanges();
+      callback();
+    };
+
+    this.start = function(component) {
+
+      const $component = $(component).html(states.initial).attr('aria-live', 'polite'),
+            key = $component.data('key');
+
+      $component
+        .on(
+          'click', '.api-key-button-show', () =>
+            $component.html(
+              document.queryCommandSupported('copy') ?
+                states.keyVisible(key) : states.keyVisibleBasic(key)
+            )
+        )
+        .on(
+          'click', '.api-key-button-copy', () =>
+            this.copyKey(
+              $('.api-key-key', component)[0], () =>
+                $component.html(states.keyCopied)
+            )
+        );
+
+    };
+  };
+
+})(window.GOVUK.Modules);