Add GOVUK Frontend footer component

Replaces the following blocks/variables with the
`footer` block filled by the `notify_footer`
macro:
- footer_top
- footer_support_links

Our current footer has a few differences
with the GOVUK Frontend one:
1. the columns of links and the meta links have no
  headings, visible or structural
2. the line of text saying GDS built this prefixes
  the meta links whereas theirs sits on a new
  link, after them

app/templates/admin_template.html

@@ -1,5 +1,6 @@
 {% extends "template.njk" %}
 {% from "components/banner.html" import banner %}
+{% from "components/notify-footer.html" import notify_footer %}
 
 {% block headIcons %}
   <link rel="shortcut icon" sizes="16x16 32x32 48x48" href="{{ asset_url('images/favicon.ico') }}" type="image/x-icon" />
@@ -124,54 +125,93 @@
   {% block fullwidth_content %}{% endblock %}
 {% endblock %}
 
-{% block footer_top %}
-  <div class="footer-categories">
-    <div class="footer-categories-wrapper">
-      <div class="grid-row">
-        <div class="column-one-quarter">
-          <ul>
-            <li><a href="{{ url_for('main.support') }}">Support</a></li>
-            <li><a href="https://status.notifications.service.gov.uk">System status</a></li>
-            <li><a href="https://www.gov.uk/performance/govuk-notify">Performance</a></li>
-            <li><a href="https://ukgovernmentdigital.slack.com/messages/C0E1ADVPC">Slack channel</a></li>
-            <li><a href="https://gds.blog.gov.uk/category/notify/">Blog</a></li>
-          </ul>
-        </div>
-        <div class="column-one-quarter">
-          <ul>
-            <li><a href="{{ url_for("main.features") }}">Features</a></li>
-            <li><a href="{{ url_for("main.roadmap") }}">Roadmap</a></li>
-            <li><a href="{{ url_for("main.trial_mode_new") }}">Trial mode</a></li>
-            <li><a href="{{ url_for("main.message_status") }}">Message status</a></li>
-            <li><a href="{{ url_for("main.security") }}">Security</a></li>
-            <li><a href="{{ url_for("main.terms") }}">Terms of use</a></li>
-          </ul>
-        </div>
-        <div class="column-one-quarter">
-          <ul>
-            <li><a href="{{ url_for("main.pricing") }}">Pricing</a></li>
-            <li><a href="{{ url_for("main.how_to_pay") }}">How to pay</a></li>
-          </ul>
-        </div>
-        <div class="column-one-quarter">
-          <ul>
-            <li><a href="{{ url_for("main.documentation") }}">Documentation</a></li>
-          </ul>
-        </div>
-      </div>
-    </div>
-  </div>
-{% endblock %}
+{% block footer %}
+  {% set meta_items = [
+    {
+      "href": url_for("main.privacy"),
+      "text": "Privacy"
+    },
+    {
+      "href": url_for("main.cookies"),
+      "text": "Cookies"
+    }
+  ] %}
 
-{% block footer_support_links %}
-  <nav class="footer-nav">
-    Built by the <a href="https://www.gov.uk/government/organisations/government-digital-service">Government Digital Service</a>
-    <a href="{{ url_for("main.privacy") }}">Privacy</a>
-    <a href="{{ url_for("main.cookies") }}">Cookies</a>
-    {% if current_service and current_service.research_mode %}
-    <span id="research-mode" class="research-mode">research mode</span>
-    {% endif %}
-  </nav>
+  {% if current_service and current_service.research_mode %}
+    {% set meta_suffix = 'Built by the <a href="#" class="govuk-footer__link">Government Digital Service</a><span id="research-mode" class="research-mode">research mode</span>' %}
+  {% else %}
+    {% set meta_suffix = 'Built by the <a href="#" class="govuk-footer__link">Government Digital Service</a>' %}
+  {% endif %}
+
+  {{ notify_footer({
+    "navigation": [
+      {
+        "columns": 4,
+        "items": [
+          {
+            "href": url_for('main.support'),
+            "text": "Support"
+          },
+          {
+            "href": "https://status.notifications.service.gov.uk",
+            "text": "System status"
+          },
+          {
+            "href": "https://www.gov.uk/performance/govuk-notify",
+            "text": "Performance"
+          },
+          {
+            "href": "https://ukgovernmentdigital.slack.com/messages/C0E1ADVPC",
+            "text": "Slack channel"
+          },
+          {
+            "href": "https://gds.blog.gov.uk/category/notify/",
+            "text": "Blog"
+          },
+          {
+            "href": url_for("main.features"),
+            "text": "Features"
+          },
+          {
+            "href": url_for("main.roadmap"),
+            "text": "Roadmap"
+          },
+          {
+            "href": url_for("main.trial_mode_new"),
+            "text": "Trial mode"
+          },
+          {
+            "href": url_for("main.message_status"),
+            "text": "Message status"
+          },
+          {
+            "href": url_for("main.security"),
+            "text": "Security"
+          },
+          {
+            "href": url_for("main.terms"),
+            "text": "Terms of use"
+          },
+          {
+            "href": url_for("main.pricing"),
+            "text": "Pricing"
+          },
+          {
+            "href": url_for("main.how_to_pay"),
+            "text": "How to pay"
+          },
+          {
+            "href": url_for("main.documentation"),
+            "text": "Documentation"
+          }
+        ]
+      }
+    ],
+    "meta": {
+      "items": meta_items,
+      "html": meta_suffix
+    }
+  }) }}
 {% endblock %}
 
 {% block bodyEnd %}