Remove edx-platform dependency on @edx/brand-edx.org

[davidjoy]

As a side effect of the way that edx-themes are loaded in edx-platform, we were required to add @edx/brand-edx.org as an explicit dependency in edx-platform when performing a rebranding of the edx.org site.

This is because a theme has no build of its own and cannot have any npm dependencies - it relies on the platform's node_modules, therefore any dependencies needed by the theme have to be added to the platform itself.

This task is to clean up this issue by finding a way to either give edx-themes dependencies or update the build process to allow brand substitution like we do with MFEs. This task will require a bit of discovery to figure out an appropriate way to accomplish this.