One-button filter downloads for non-Gmail clients

[amyxzhang]

It would be great to have a way, in the documentation for setting up filters to forward email to Squadbox, a one-click button to download filters for each major email client (Apple Mail, Outlook, Yahoo, etc.) that is not Gmail.

At first, we can just start with the vanilla filter of forward everything except for those with the list-id.

We need to test if this violates DMARC for any of these clients.