mailchimp 6.x-2.0-beta2

I jumped the gun a bit on the last beta release, this one is much better!

• adding mailchimp namespace to constants
• added constants for listtypes
• merged profile and registration forms into a single form_alter and added a submit handler for registrations
• adding an install file to handle the deletion of unused variables
• implementing interest groups at the account and registration forms, #572114: Interest Groups on user registration page
• add an admin setting for showing interest groups on registration and account forms