Add customUrlPath config param for server task

**RFC - probably don't merge**

We would like to be able to emulate our production paths as closely as possible in development. What we want is to mount the app at something like `/development/engagement`, but still hit the back-end API at
`/development/api/...`

Using the `relativeUrlRoot` option, I have not been able to figure out a way to do this. It appears that the `relativeUrlRoot` option is used both by the api proxy *and* by the express server, and therefore it will only work if we want the same relative path for both servers.

This works, but I am aware that with my complete lack of "node/express"-fu, this is quite possibly the WRONG way to do this, so I'm asking for input. Is this reasonable? Or am I completely missing something obvious in the application config options?