refactor: simplify the course-home tour button (#1993)

Remove the vestigial `metadataModel` prop from the shared TabPage. Its only live
effect was gating the screen-reader-only launch-tour button, which is only ever
real on the outline tab — gate that on `activeTabSlug === 'outline'` instead and
rename the helper to `renderSrOnlyTourButton`. Drop `metadataModel` from
LoadedTabPage and its dead pass-through to StreakCelebrationModal, and from the
DatesTab / CoursewareContainer / TabContainer call sites. Move
LaunchCourseHomeTourButton's `courseId` from the Redux slice to `useParams`.

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