Add configurable tags to the webform handler configuration screen.

This patch adds a textfield to the webform handler that accepts one or more tags (separated by comma's). It adds the tags to the mailchimp_subscribe() function used in the handler code.

Do note that when adding tags to sign-ups this way, Mailchimp will create new tags for the audience in case the tags were not yet created on the audience before.

This is a super simple quick fix that does what we need it to do.

I see there is a related issue that's even older than this one.

With the way this is coded, any actual problems with the tags being attached would need to be handled in the Mailchimp module → upstream. I personally think this one is good to go, and is workin in my production instance at https://mcgreenacres.com/form/contact.

I re-rolled the patch to make it work with version 8.x-5.11 of webform_mailchimp.