Issue 6: textarea character limitations.

HTML has a maxlength property that can be set to textareas, here they're
set to 255 which is what MailChimp is limiting us to anyway.

For visual, a small jQuery function counts the characters in the
textarea and displays how many are left to the user.

I think this is a temporary measure for issue #6 since it seems we actually want more than 255 characters so I won't mark this as a fixing pull request.