Move special case to whatdotheyknow-theme

Apart from anything else, we don't want translators to have to worry
about the special case text. See https://github.com/mysociety/whatdotheyknow-theme/commit/2078febca5181ce3b1a9c0fae0123ae5f6448718 for the corresponding change to whatdotheyknow-theme.
Merge branch 'feature/email-subject-cleanup' into rails-3-develop