Mulit-site Download Agreement Fix

Fixes #175. The Mezzanine settings which are editable such as `DOWNLOAD_AGREEMENT_VERSION` are managed per site. This means the English and Chinese versions of the site can have different version numbers. This change ensures that the version number checked against for signing and displaying is always the `DOWNLOAD_AGREEMENT_VERSION` from the English site.
As I mentioned on IRC, I really wish there were a cleaner way of doing this, and may look deeper into alternatives later, but this works for now.