Add footer to email newsletter

[tomgenoni]

Doing this so I don't have to inject CSS/JS to format the page for sending emails. This means we can just copy directly from the newsletter page and paste it into a Gmail compose window.

I've hardcoded the footer info into https://github.com/thumbtack/thumbprint/blob/tom/email-footer/www/src/pages/updates/notes/2019-05-07.mdx for an example.

We can either let it render like that or hide it behind a query param.