Use relative publicPath in webpack build

[haynesjm42]

Have a use case where a client app needs to be hosted under a sub-path (https://client.com/subPath/clientApp) and using / as the base publicPath causes issues. We should be able to just set it to ../ for prod builds and everything will work the same since the index.html is always in an app sub-directory from the root context.

[amcclain]

Closed via #7