[dagit] Repair breadcrumb links in TopNav
Summary: Breadcrumb links in the top nav currently use Blueprint's default link rendering, which is not a react-router link. Fix this so that breadcrumb navigation doesn't constantly reload the page.
Test Plan: View Dagit, navigate via breadcrumbs. Verify proper link, hover, etc. rendering and behavior.
Reviewers: bengotow, sashank, prha
Reviewed By: sashank
Differential Revision: https://dagster.phacility.com/D5411