Suppress Jetty directory listings

This ought to have been done at the same time as
24152dd
but I overlooked it. At any rate both the source code and built image
for isaac-api are public, so this would not list anything sensitive.

web-api-live.xml

@@ -57,6 +57,10 @@
             org.jboss.resteasy.plugins.interceptors.GZIPEncodingInterceptor
         </param-value>
     </context-param>
+    <context-param>
+        <param-name>org.eclipse.jetty.servlet.Default.dirAllowed</param-name>
+        <param-value>false</param-value>
+    </context-param>
 
     <session-config>
         <cookie-config>