Subscribing to our newsletter on NoRent's footer requires JS

[toolness]

In #1556 we have improved the UX for subscribing to our mailing list, but the one downside is that it now requires JS. It's pretty core functionality, though, so we should ideally provide some fallback mechanism.

One interesting thing is that it's actually already possible to make a standard HTML <form> that just POSTs to /mailchimp/subscribe--in fact, it's how the API sandbox in the endpoint's documentation page (accessible at the same URL that one POSTs to) works. The only problem with it is that it just returns a JSON response, which isn't terribly helpful--so if we instead e.g. use the Accept header to return a human-readable response, that might do the trick... Anyways, just a thought.