Increase the default maximum form content size on Jetty

The default jetty settings restrict form data size to 200 000 bytes, which can be a bit too restrictive. We can set this to be unlimited, but I think this can cause security problems with DoS attacks. If users need to post larger attachment, they can manually increase the size. See http://www.xwiki.org/xwiki/bin/view/FAQ/FormTooLargeError

I'd say that 1M is enough as a default value.