Allow specifying the base path for links and such

[mohkale]

I'm trying to proxy through to netviel from nginx with HTTPS authentication as well.

The base path for the server is /mail so /mail/* is forwarded to netviel since the index.html references net-viel.js the request for that isn't being resolved since its within the /mail namespace. It would be nice if there way someway to specify a path prefix for asset paths so they can be resolved properly.