Feature request: custom URL

[appolloford]

Following the discussion in PR #1307, we agree to some extent that it is good for CARTA to support custom URL. This function is particularly helpful for setting a reverse proxy (e.g. via Open OnDemand) for CARTA and is also implemented in Jupyter (ServerApp.base_url) and Matlab (MWI_BASE_URL). Similar to the two softwares, we propose to have CARTA support adding URL prefix behind the host:port for proxy to parse.