Tango Server Workspace

Motivation: We have the first remote workspace, but we still feel mixed about it. Also, it's not useful for people that don't use WandB. This is a different workspace that needs only a connection to a running Tango server.

A Tango Workspace keeps track of step results and a bit of metadata for a step. We have two implementations so far. One implementation keeps all the data in a local directory. Another keeps it in WandB. This would be a third implementation that uses a REST API to handle the data. It means writing both the client side (a `Workspace` implementation) and the server side (no restrictions but it has to be Python). The storage backend would likely be whatever the Google Cloud offers, since that's what we use. The resulting server should be deployable through Skiff.