DEV: Adds script for Mailchimp mailing list management

[Ahmedgagan]

This PR adds a Script named Add to Mailchimp Mailing list to the Automation plugin, this Script will be triggered when a new user is created or any user is updated, allowing the admin to provide users with an option to add/remove themselves from the specified Mailchimp mailing list.

When the admin creates a Mailchimp mailing list script, it adds a checkbox on the signup page and user preferences profile page. Once the user clicks on the checkbox and signs up or saves preferences he is added OR removed from the mailing list of Mailchimp specified in script options.

Also, I have created a webhook for Mailchimp that will keep sync between Mailchimp and discourse when the admin adds any user to the mailing list from Mailchimp Dashboard.