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/revoke.html

@@ -1,31 +0,0 @@
-{% extends "withnav_template.html" %}
-{% from "components/page-footer.html" import page_footer %}
-{% from "components/api-key.html" import api_key %}
-
-{% block service_page_title %}
-  Revoke API key
-{% endblock %}
-
-{% block maincolumn_content %}
-
-    <h1 class="heading-large">
-      Revoke API key
-    </h1>
-
-    <p>
-      ‘{{ key_name }}’ will no longer let you connect to GOV.UK Notify.
-    </p>
-    <p>
-      You can’t undo this.
-    </p>
-
-    <form method="post">
-      {{ page_footer(
-        'Revoke this API key',
-        back_link=url_for('.api_keys', service_id=current_service.id),
-        back_link_text='Back to API keys',
-        destructive=True
-      ) }}
-    </form>
-
-{% endblock %}