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This introdues the idea of "upstream" and "downstream" content, where downstreams (like course components) can pull content updates from upstreams (like learning core-backed content library blocks). This supports the upcoming Content Libraries Relaunch Beta for Sumac. New features include: * A new XBlockMixin: UpstreamSyncMixin. * A new CMS Python API: cms.lib.xblock.upstream_sync * A new CMS JSON API: /api/contentstore/v2/downstreams * A temporary, very basic UI for syncing from Content Library blocks Implements: https://github.com/kdmccormick/edx-platform/blob/kdmccormick/upstream-proto/docs/decisions/0020-upstream-block.rst Co-authored-by: Braden MacDonald <braden@opencraft.com>