Separate edX / Open edX footer.

We need to use an Open edX footer by default (and that's what all non-edx.org
instances should base their footer off) and only use the edX footer on
edx.org.

This commit uses `SITE_NAME` to determine which footer to use.

Sites using theming or micro-sites with a `SITE_NAME` not equal to `edx.org`
should not be affected.

Note that this is still a work in progress as we will eventually use logos
hosted on S3.