Quotations incorrectly escaped in signup form

The signup form used both at /mailchimp/subscribe and in the signup blocks incorrectly escapes quotations.
For example, a list called "foo'bar" would present the text "Sign up for foo'bar".

This happens because the text is escaped using check_plain(), which in turn calls htmlspecialchars($text, ENT_QUOTES, 'UTF-8').
Doing this here requires decoding the quotation marks again afterwards.