(TK-33) Add redirect support to proxying

This PR adds proper redirect support to proxying in the Jetty9 service. 
I believe this test as currently written will pass because the "/goodbye/world" path on the target server does not exist when the ProxyServlet follows the redirect from "/hello" on the target server to get to it.  Jetty returns the 404 on its own in this case -- not the ring handler.

I think this test would be more interesting as a "success" case.  I'd like to see that the ProxyServlet is able to successfully follow a redirect from the target server to a location that is not under the original proxy path.  I was able to achieve this by adding a second Ring handler to the target server in `with-target-and-proxy-servers`:

```clj
 (add-ring-handler
         target-webserver#
         ~ring-handler
         "/goodbye")
```
I then just changed the assertions in the test to look for a 200 status and "Hello, World!" body.

What do you think?