fix bugs with DevAppServer and EE8

[lachlan-roberts]

• “IllegalStateException: Committed” after 403 Forbidden sent by DefaultServlet when directory listing is now allowed.
• Investigate and fix classloading issues not being able to load jetty-servlet classes from the web.xml configuration.
• Fix the redirect loop issue in the LocalResourceFileServlet.
• Use the startWebApp() method introduced in 12.0.8 for EE8 AppEngineWebAppContext.
• NullPointerException caused because the HtmlEscaper rejects null value for the error message from the ResponseRewriteFilter.
• Fix java.lang.IllegalStateException: Response has already been committed, because the ResponseRewriterFilter.ResponseWrapper was committing the response twice.

[ludoch]

We will need a regression test both for EE 8 and EE 10.

[ludoch]

Also for the record, the regression was introduced in https://github.com/GoogleCloudPlatform/appengine-java-standard/pull/105/files

[ludoch]

We will wait for the regression test, approving only for nighty build