Adds an X-I2P-Location header filter to the default I2P site

This adds a filter to the default site which, when it is accessed from a non-I2P URL, replies with an X-I2P-Location=http://thesitesbase32addresswouldgohereinarealonebutnotnow.b32.i2p/the/rest/of/the/path header. If the I2P User changes the configuration to listen on all addresses, or connects the jetty server to a reverse proxy, the header will then be added to the response, allowing a browser configured to recognize the header to inform the user of the alternate location. At this time I2P In Private Browsing(Firefox) supports the header, and I will add it to i2pchrome.js(Chromium, Firefox for Android) in the next release.

There is no UI to configure the header. A non-i2p request URI will always result in the header being included in the response to the browser, including when visiting http://127.0.0.1:7658. The browser extensions currently/will continue to only recognize the header and provide the user with an option if served over HTTPS.

The header could also be recognized by the HTTP Proxy and used to offer the user an interstitial page when visiting HTTPS based non-anonymous sites via an outproxy, but that is a MR for another time.