Process mailchimp forms based on class name instead of id

[slackero]

I had the problem to use the Mailchimp extension twice on a page (constant in the footer and inside content). It always resulted in DOM errors – because id="mailchimp-form" isn't unique in this case.

Because I did not find a simple solution to load another template (Templates/Form/Index.html) to just use a completely different structure I have refactored the JavaScript.

Bit off-topic: Would be great to have an option to select another template or just have access to the outer tt_content values. I thought to use a custom tt_content.layout value as condition but it's not available when the mailchimp form is rendered.