Grant access for important docs based on Slack channel membership

I just realized that many organizers didn't and don't have access to our password doc.

Since being invited to #organizing-priv as a mark of trusted membership, we could use this to grant access, assuming people use their Google-friendly email for our Slack.

When someone joins a specific private channel `#organizing-priv`, it would be great to automatically grant access.

- this could run nightly
- it should announce that it's added people in `#organizing-priv` and `#organizing-open`, so it's not forgotten that this is happening
- would be great if it was on-demand right after adding, but that's a little harder, so maybe later.

Docs to grants:

- Edit access on GDrive folder hierarchy (normally publicly viewable)
- Edit access on passwords GDoc (normally private)