Copy suggestions for List Group enhancement

[cmsteadman]

In #122 - we added support for List Group. Our technical content team has some suggestions for the copy associated with this feature.

Intro Text: If you set up groups in MailChimp, include them in your checkout page so customers can choose the type of email marketing they want to receive.

Current Copy: Interest Group - Suggested Copy: Available Groups

Current Copy: Interest groups in checkout success - Suggested Copy: Show groups after checkout?

Current Copy: Success message before - Suggested Copy: Group Description

Current Copy: Success message after - Suggested Copy: Success message

In Customer Information > MailChimp, Mailchimp Information casing should be MailChimp Information