[dagit] Crag: Flattened left nav jobs

Summary: A first pass at flattening the left nav to display jobs, with iconography to indicate schedule/sensor status, as discussed.

Test Plan: Enable flag, verify that jobs appear in the left nav, with icons.

Reviewers: bengotow

Reviewed By: bengotow

Differential Revision: https://dagster.phacility.com/D8363