Fix grqphql playground asset paths
Grqphql playground assets were being copied to /webapp/vendor instead of /webapp/build/vendor. This fixes the copy to the right path.
Also removes /webapp entirely as a build step in rebuild_dagit so that we shouldn't have this issue again
Test Plan: open localhost:3333/grqphql and it loads
Reviewers: #ft, max
Reviewed By: #ft, max
Differential Revision: https://dagster.phacility.com/D635