Add no-click starter-starter for binder, etc.

[bollwyvl]

Elevator Pitch

Add a way to open a starter immediately on Lab launch without any user interaction.

Motivation

Sometimes, one click is still too much to introduce something in lab, compounded by starters appearing "below the fold" in smaller windows due to JupyterLab Launcher style choices.

Design Ideas

Add a router, such that a binder URL can be created like ?urlpath=lab/starter/<starter>/<path> and immediately open the sidebar.