Moving annotation rows always changes the new item's rating population, so
the recompute belongs with the move rather than at each call site. Covers
the album reassign in the remap and the two scanner sites, and replaces the
explicit call ReassignReferences was making.
Album was the worse case: rate an album, move its files, and 'missing fix'
handed the rating to an album still caching an average of 0.
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