[dag-star] index html and static resources

Summary:
Add handlers to serve index.html and all the static resources

depends on D8807

Test Plan:
Manual:
launch with uvicorn dagit.starlette:default_app --ws websockets --port 3333
vist localhost:3333/ verify dagit loads, server does not show errors for static resource requests

Reviewers: prha, max, bengotow, dish

Reviewed By: prha

Differential Revision: https://dagster.phacility.com/D8809