Update mail chimp integration to sync membership status to contacts

[jabelone]

Right now MM automatically creates a new mail chimp contact and tags them with "member" when they sign up on MM. However, some members don't actually make it through the full signup process so we end up with some mail chimp contacts tagged as members who aren't actually active members. We should change this tag to "member-signup" to make this more clear, and sync the membership type and membership status to that mail chimp contact when they change (for example when a subscription starts or ends).