This commit adjusts a few values in our discussions configuration APIs to make them match what the frontend needs, as well as to more accurately reflect the providers available today.
- The `active` provider ID is expressed as None if it doesn’t exist
- The “cs_comments_service” provider has been renamed “legacy” - when we implement the new discussions micro-frontend, we’ll also have a separate provider for that, so they can’t both be “cs_comments_service”. Also, cs_comments_service is such a bad name for anything.
- The hard-coded providers list in get_supported_providers now includes ‘legacy’ and ‘piazza’, our two known providers. This list will be updated as more known providers come online.
- The PROVIDER_FEATURE_MAP has similarly been updated.
Part of this task: TNL-8093
