[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