(TK-33) Add redirect support to proxying

This PR adds proper redirect support to proxying in the Jetty9 service. 
@fhrbek With `setFollowRedirects` set to false (the default), any redirects that the ProxyServlet receives from a target server should be sent back up to the original client.  The main problem we were debating was whether the client would have the right context to follow that redirect on its own or whether instead we'd need to "munge" the location header in order for the client to follow the redirect back through the proxy server.  For example, it may be that the server on which the proxy is running has appropriate access to follow the redirect but the client would not (e.g., due to firewall issues, ...).  Where these kinds of issues aren't in play, a `setFollowRedirects` of false on its own should be sufficient for allowing the redirects to be client-driven.  With the *munging* approach on the server-side, however, we were finding that there were a couple of scenarios where we couldn't come up with an appropriate route to guide the client on the redirect.  For more information on that, you might take a look at some of the comments we listed in the JIRA TK-33 ticket.  We'd definitely welcome any additional feedback you might have on the whole *munging* idea, assuming we end up pursuing that later on.