Reorganise templates for API keys page

This adds another layer of folder structure to the templates to match
the new URLs, which have another layer of hierarchy.

app/templates/views/api/keys/show.html

@@ -0,0 +1,28 @@
+{% extends "withnav_template.html" %}
+{% from "components/page-footer.html" import page_footer %}
+{% from "components/api-key.html" import api_key %}
+
+{% block page_title %}
+  New API key – GOV.UK Notify
+{% endblock %}
+
+{% block maincolumn_content %}
+
+
+    <h1 class="heading-large">
+      New API key
+    </h1>
+
+    <p>
+      Copy your key to somewhere safe. You won’t be able to see it again
+      once you leave this page.
+    </p>
+
+    {{ api_key(secret, key_name) }}
+
+    {{ page_footer(
+      secondary_link=url_for('.api_keys', service_id=current_service.id),
+      secondary_link_text='Back to API keys'
+    ) }}
+
+{% endblock %}