[dagit] Fix mode-less pipeline paths
Summary: There is an infinite loop when loading a pipeline URL that does not have a mode. Repair this by being more restrictive about our effect dependencies.
Test Plan: Load http://localhost:3000/workspace/toys_repository@dagster_test.toys.repo/pipelines/asset_lineage_pipeline, verify redirect to :default and no infinite loops.
Reviewed By: bengotow
Differential Revision: https://dagster.phacility.com/D8247