New check: auto-delete merged branches

[jmbowman]

We generally prefer to delete feature branches shortly after the associated pull request is merged, and there's a per-repository GitHub setting to do this automatically. Add a new health check to report on the status of this setting, and add it to the console & streamlit dashboard configuration.

The conversation that prompted this issue: https://openedx.slack.com/archives/C0497NQCLBT/p1704728188303909