Refactored MailChimpExport.cs to use ServiceStack to create URL Request

Previously MailChimpExport.cs used a custom method, ReturnFullUrlWithParameters, to create the full MailChimp URL.

This was rewritten to use ServiceStack, which allowed the custom ReturnFullUrlWithParameters function to be eliminated.

The _httpsUrl string was updated to include a trailing slash, which prevents a HTTP redirect when used with a POST request. This redirect did not include the original data POSTed to the API endpoint, and would therefore fail with a no API key error.