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/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 %}