Consider a web interface to set parameters for server mode

[pont-us]

When running an xcengine container in server mode, parameters for user code have to be passed as arguments to the Docker image's entry point. In many deployment scenarios, this may not be practical.

Idea: add an option for server mode whereby the server and viewer aren't started at once. Instead, a single-page web configuration interface comes up, allowing the user to set the parameters before running the notebook code and starting the server.

Could also provide a REST endpoint for the same purpose, though I'm not sure that there's a compelling use case for that.