Add email capture form to blog posts

[benjaminhoffman]

What's our thoughts on starting to collect emails on Gatsby's blog? This is a janky proof of concept but I'd be happy to continue hacking through it if we like this...

The code is not pretty but I can open a PR. Curious how / where to store the email addresses (db + mailchimp?); it would be great to track which URL the email was collected on + track conversions in GA.

cc @shannonbux

[KyleAMathews]

This would be great! MailChimp would be simplest I think since we'd use that for sending out emails.

[benjaminhoffman]

PR: #3333