Features/mailchimp integration

[billybonks]

Hi, this repo was super helpful when creating our prelaunch website. we wanted to integrate with MailChimp.

I added the following features.

• Instead of sending the welcome mail from rails, it gets sent from MailChimp
• Users when users subscribe on MailChimp a webhook updates their User records as verified.

The feature is completely backward compatible, since it is only activated if the developer adds their MailChimp variables to the env file.

I think it would be worthwhile to add it to the repo incase someone wants to achieve the same thing.