Add styles and Jinja for component

Most of these are cut-and-paste'd from the GOVUK
metadata component:

https://components.publishing.service.gov.uk/component-guide/metadata

...but stripped back to only include what we need
and a optional suffix paragraph added.

Links to styles and ERB template
https://github.com/alphagov/govuk_publishing_components/blob/master/app/assets/stylesheets/govuk_publishing_components/components/_metadata.scss
https://github.com/alphagov/govuk_publishing_components/blob/master/app/views/govuk_publishing_components/components/_metadata.html.erb

Also adds it to the accessibility statement as an
example and updates the test that checks any
updates bump the date.

app/templates/components/content-metadata.html

@@ -0,0 +1,13 @@
+{% macro content_metadata(data, suffix=False) %}
+<div class="content-metadata">
+  <dl>
+  {% for key, value in data.items() %}
+    <dt class="content-metadata__term">{{ key }}</dt>
+    <dd class="content-metadata__definition">{{ value }}</dd>
+  {% endfor %}
+  </dl>
+  {% if suffix %}
+  <p class="content-metadata__suffix">{{ suffix }}</p>
+  {% endif %}
+</div>
+{% endmacro %}