[dagit] PipelineExecutionRoot refactor
Summary: Refactor queries in PipelineExecutionRoot, which will make it simpler to implement a followup in which we prepopulate the config editor when there is a single preset.
Test Plan: View Playground for pipeline, verify proper rendering and behavior of the page.
Reviewers: bengotow, prha
Reviewed By: bengotow
Differential Revision: https://dagster.phacility.com/D8601