[dagit] Extract ContentRoot to be an App child

Summary:
Provide ContentRoot from the index level. This allows better customization of the route tree.

I had considered making route configuration more abstract (and even landed some code for it) but it's just not worth introducing that level of complexity yet, and there are more pressing needs that this helps with.

Test Plan: Run Dagit, verify that workspace, instance, and settings routes all work correctly.

Reviewers: bengotow, prha, dgibson

Reviewed By: prha

Differential Revision: https://dagster.phacility.com/D9211