Legend: Have commonOptionsBuilder.addLegendOptions replace “stats picker” editor with nested picker - like “units picker” - to better accommodate percentile

[baldm0mma]

When the percentile calculations were added to our reducers, this issue began, and was "fixed-ish" here. This is still not a great fix, and could be handled better with a different style of value picker. Instead of listing all values for reducer calcs (including ALL 98 percentile options), we could replace the current picker with something more akin to what we see in the "units picker" which has subclasses of options in the dropdown. That way, the percentiles will only appear when their "subclass" is chosen.