Refactor pivot table components to accept bigints

[cmatzenbach]

To finish what was started in PR #394, there are three areas where we are currently accepting precision loss, which require this refactor:

1. getSortValue line 176 - sort order for non-aggregated rows with large bigints
2. getSortValue agg path - agg.sum, agg.min, agg.max come from PivotedRowAgg which stores number | null, so even though we now compute them more accurately as bigints, they're converted to number before being stored and returned here
3. gradientBg color scaling - uses the number sort value to position a value on a color gradient, so large bigints that lost precision in getSortValue get slightly wrong colors

Cascading changes are needed for gradientBg and the column range comparisons in PivotedStatsTable.tsx.