[dagit] Properly disable LaunchButton
The "disabled" state of the Run launch button doesn't get passed down to the button itself, so even if the button looks disabled, it's not. Fix this.
Test Plan: Force disabled prop on the LaunchButtonDropdown component via React devtools, verify that the button is disabled and does nothing when clicked.
Reviewers: bengotow, alangenfeld
Reviewed By: alangenfeld
Differential Revision: https://dagster.phacility.com/D5393