Provide default workspace configuration for projects

[micahjsmith]

Currently, ballet renders a workspace.json file as part of its project template that is used by projects: The workspace is then configured when binder launches. This means that Assemble cannot control its desired layout, and that instead it is in configuration files in all of the different Ballet projects.

Instead, Assemble should do the configuration to match what is currently set in the workspace.json:

• the file browser open to a specific path in the project (optional, as this would require finding the right path using the ballet.yml file, so maybe this specific configuration option could stay in the project template)
• open the analysis notebook at the configured width
• open the feature engineering guide at the configured width

I'm not sure how Assemble would do this configuration, maybe it is something that it does on extension load.