calculate_from_sample is called when clicking the General / Artist /
Copyright / Characters buttons during tag editing. Currently it's slow
because it counts up the tags in ruby, and because it makes repeated
calls to memcache in the inner loop when filtering tags by category.
This changes it to do the counting and filtering entirely in SQL using a
GROUP BY tag / COUNT(*) aggregation. This is faster and makes larger
sample sizes more feasible.
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