Add pupa clean DAG

[antidipyramid]

Overview

This PR adds a new DAG called clean_stale_db_objects to clean stale items from the database using the pupa clean command. I've set it to run once per week.

Checklist

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

Testing Instructions

• Verify that you can manually run the clean_stale_db_objects DAG in the Airflow UI

[hancush]

@antidipyramid Can you change the base of this branch to the upgrade branch, so that only your changes are in the diff? 😇

[antidipyramid]

@hancush Done!