Make clean_stale_db_objects configurable

[hancush]

Overview

See title! This PR also updates local development instructions in README.

The primary concept here is params: https://airflow.apache.org/docs/apache-airflow/2.6.3/core-concepts/params.html#params

As of 2.0, Airflow has a new concept called task flows, that are less boilerplate-y and more declarative than traditional DAGs. They also allow you to pass the return value of one task directly to another. Here's more, specifically how to declare dependencies between tasks and traditional operators, which we need to here: https://airflow.apache.org/docs/apache-airflow/2.6.3/tutorial/taskflow.html#adding-dependencies-between-decorated-and-traditional-tasks

Checklist

• PR has a descriptive enough title to be useful in changelogs

Testing Instructions

• Run the dashboard as described in the README
• Trigger a clean_stale_db_objects job with configuration. Use different values for window and max, or toggle on report, and confirm the DAG behaves as you've configured it.

Closes #140, closes #141

[antidipyramid]

Looking good! Can't wait for more DAGs to be configurable in the same way.