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);

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);
+  }
+
+}

app/assets/stylesheets/main.scss

@@ -45,6 +45,7 @@
 @import 'components/management-navigation';
 @import 'components/dropdown';
 @import 'components/email-message';
+@import 'components/api-key';
 
 @import 'views/job';
 

app/templates/components/api-key.html

@@ -0,0 +1,5 @@
+{% macro api_key(key) %}
+  <div data-module="api-key" data-key="{{ key }}">
+    <span class="api-key-key">{{ key }}</span>
+  </div>
+{% endmacro %}

app/templates/views/styleguide.html

@@ -7,6 +7,7 @@
 {% from "components/sms-message.html" import sms_message %}
 {% from "components/table.html" import mapping_table, list_table, row, field %}
 {% from "components/textbox.html" import textbox %}
+{% from "components/api-key.html" import api_key %}
 
 {% block page_title %}
   Styleguide – GOV.UK Notify
@@ -152,4 +153,8 @@
   {{ textbox(form.password) }}
   {{ textbox(form.message, highlight_tags=True) }}
 
+  <h2 class="heading-large">API key</h2>
+
+  {{ api_key('d30512af92e1386d63b90e5973b49a10') }}
+
 {% endblock %}