Add captcha to our mailing list sign up form

People are starting to have their email addresses added to our mailing list but they are not signing up themselves. I enabled double opt-in. Adding captcha would stop bots from adding them if that's happening.

Start by locating the mailing list sign-up form and reviewing how its enabled double opt-in flow currently works. The work is done when automated additions are blocked by a captcha while legitimate visitors can still complete the sign-up process.

Written by the indexing model from the issue text.