[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