Add link back to API integration from callbacks

Matches what we do on the API keys and whitelist pages.

app/templates/views/api/callbacks.html

@@ -30,4 +30,8 @@
 	    {% endcall %}
 	  {% endcall %}
 	</div>
+  {{ page_footer(
+    secondary_link=url_for('.api_integration', service_id=current_service.id),
+    secondary_link_text='Back to API integration'
+  ) }}
 {% endblock %}