Add "Check for Spam" button to Feedbacks

Just like Akismet does for comments, allow users to recheck their Feedbacks queue for spam. This is useful in the case that Akismet misses the first wave of a spamflood, allowing the user to clear out possibly thousands of submissions that would now be caught as spam.
Couldn't this line be more easily written as `if ( isset( $_POST['limit'], $_POST['offset'] ) ) {` ?