Use confirmation banner for revoking API keys

Currently revoking an API key takes you to a separate page. It should
work the same way as other destructive actions, ie staying on the same
page but with a banner asking you to confirm the action.

app/templates/views/api/keys.html

@@ -1,4 +1,5 @@
 {% extends "withnav_template.html" %}
+{% from "components/banner.html" import banner_wrapper %}
 {% from "components/table.html" import list_table, field, hidden_field_heading %}
 {% from "components/api-key.html" import api_key %}
 {% from "components/page-footer.html" import page_footer %}
@@ -9,6 +10,22 @@
 
 {% block maincolumn_content %}
 
+  {% if revoke_key %}
+    <div class="bottom-gutter">
+      {% call banner_wrapper(type='dangerous', subhead='Are you sure you want to revoke this API key?') %}
+        <p>
+          ‘{{ revoke_key }}’ will no longer let you connect to GOV.UK Notify.
+        </p>
+        <form method='post'>
+          <input type="hidden" name="csrf_token" value="{{ csrf_token() }}" />
+          <input type="submit" class="button" name="delete" value="Confirm" />
+        </form>
+      {% endcall %}
+    </div>
+  {% else %}
+
+  {% endif %}
+
   <div class="grid-row">
     <div class="column-two-thirds">
       <h1 class="heading-large">