[dagit] Put SolidsRoot under workspace nav
Summary: The Solids root page currently has no top nav, but it should live under the workspace-level nav like the Pipelines and Schedules lists.
Test Plan: View /solids on a workspace, verify that the nav appears above the Solids view, and that the links in the nav work as expected. Click around within the Solids list, verify proper behavior.
Reviewers: bengotow, prha
Reviewed By: bengotow
Differential Revision: https://dagster.phacility.com/D5204