fyi.opensocial.subscribeSpaces

Changes to a group's spaces, as they happen, for one of its members: a #change for each change to a space the caller may read now, saying which space and its new revision, and nothing of what changed. A member who hears one reads again with listSpaceRecords. Members only, with service auth for this method, sent as a Bearer token or, as a browser's WebSocket can't send one, as `token`. The site's own pages hear the same changes at /space-events (server-sent events, with who's online).