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.