Handle double opt-in when user has attempted signup before

[houmark]

I submitted a PR to improve the handling of signups. More details in the pull request itself.

Basically we saw that when someone attempted signup but didn't finish by confirming in the mail, because the mail was not delivered for some reason, it would be impossible for that e-mail to get on the list through this extension on a later attempt.

After chatting with MailChimp tech support and reading up on their docs, the solution was to either do a PUT request on the first signup attempt, or catch the "existing membership" when doing the POST and then do a PUT request.

Since a PUT request will also change subscribed to pending for an existing user the best solution is to do a POST request, catch the "error", do a GET request for the current status of the mail (the POST does not provide this information) and if that status is not "subscribed" then do the PUT request to set the user to pending. When that happens, MailChimp will automatically send the confirmation mail again. This 3-step routine prevents an existing user from changing from "subscribed" to "pending" and we will still output the "you are already signed up" message.

If the user is not on the list in any way, only one POST request is sent and that one will of course also trigger the confirmation mail.

Hope to see this merged soon :)