[dagit] Extract app configuration
We currently pull a handful of static configuration values out of global space in Dagit, at various points throughout the app.
Instead, provide these at a single point (index.tsx) and make them available via context. This gives us a place to define top-level configuration that can be made more flexible over time.
Also create the Apollo client within the scope of the AppProvider, feeding it configuration as needed.
Test Plan: BK, load Dagit and sanity check by navigating through the app.
Reviewers: bengotow, max, dgibson
Reviewed By: bengotow
Differential Revision: https://dagster.phacility.com/D7197