Add resubscribe capability and allow multiple values

Problem/Motivation

When a user unsubscribe himself from a newsletter and later on resubscribe himself through a webform, then a flexmail-error 'address is blacklisted' will occure.
And when using a select list in the webform and it's fields is mapped to a flexmail field, we got the flexmail-error that we must send an array instead of a string.

Steps to reproduce

Proposed resolution

With this patch an extra configuration item is added 'optin_forms', so when we receive an 'address is blacklisted' and a 'optin_forms' is selected, then a new /optin call is send to flexmail. So the user receive a mail from flexmail with an optin-form.
And by change the allow_multiple_values to 'TRUE', the selected items will be send as an array.

Remaining tasks

User interface changes

API changes

Data model changes