Add pop-up for Mailchimp mailing list sign up after exploring FTL functionality

[burrage]

We want to pop up a modal requesting an e-mail to join the mailing list similar to the pop-up on curiouslearning.org:

Requirements:

• Those that have signed up already shouldn't receive the pop-up.
• Those that decline shouldn't see it again for 30 days.

[burrage]

We ended up using an exit pop up modal that is controlled within Mailchimp.