Move uuid helpers to out of the SiteUUID model

[OmarIthawi]

Move the organization mapping @classmethod helpers to be independent helpers, this way we can do the following:

def get_organization_by_uuid(organization_uuid):
    if should_site_use_org_models():
        return Organization.objects.get(edx_uuid=organization_uuid)
    else:
        TahoeSiteUUID.objects.get(site_uuid=organization_uuid).organization

Such helpers will enable abstracting the migration from LMS/CAG/Figures since they'll reference the same helper and their code will be unchanged.

Originally posted by @OmarIthawi in #2 (comment)