[dagit] Allow configuration of target path for app.py
Summary: Add a parameter to app initialization for Dagit, allowing a target dir to be specified. This is useful in cases where the app is being initialized from a source other than the current directory.
Test Plan: Buildkite. Run dagit, sanity check that it works.
Reviewers: dgibson, max
Reviewed By: dgibson
Differential Revision: https://dagster.phacility.com/D7171