refactor: convert the discussion tab from Redux to React Query (#2005)

Self-wrap the discussion tab (metadata-only: no tab-data fetch, so TabPage's
tabDataQuery becomes optional). Also convert CourseAccessErrorPage — the other
fetchDiscussionTab caller — to useCourseHomeMeta, letting fetchDiscussionTab be
deleted. Ordered below the live tab so the shared fetchTab helper retires cleanly
there. Part of #1975; closes #2003.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>