(TK-33) Add redirect support to proxying

This PR adds proper redirect support to proxying in the Jetty9 service. 
Yeah, but that was in the context of the proposed location header *munging* solution.  For that approach, the redirect would have to be triggered by the client back through the ProxyServlet to a target server.  When the target is outside of the proxy path that is "hard-coded" into the ProxyServlet, the ProxyServlet would not have the context to know where to proxy the redirect request to.  We thought it would be better, then, to just have the ProxyServlet return an error back instead of the original redirect response in order to more explicitly highlight the problem.

With the approach where the ProxyServlet is handling the redirect on its own rather than depending upon the client to do it, this same problem doesn't exist.  The ProxyServlet should just follow the location header returned from the target server and be able to navigate to it properly even if the target of the redirect is not under the same path as was the original target.

I'm thinking we should code this test for the capability that the ProxyServlet should be able to offer here.

This test will have to look different if/when we were to implement the full location header munging w/ client redirection approach since we'd have to code for the failure logic.

Does this make sense?