[dagit] Fix favicon for path-prefix case
Summary: The favicon path is broken for the path-prefix case. Fix this by restoring the link tag in index.html so that it can be modified by app.py, and update the custom run-status favicon paths using the basePath value.
yarn build-for-python, run dagit with and without path-prefix. Verify that favicons load correctly, and update when navigating to runs.
Run in dev mode too, verify that everything works.
Reviewers: bengotow, prha
Reviewed By: prha
Differential Revision: https://dagster.phacility.com/D7567