investigate duplication identified by jsinspect

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Code review #52, checklist item:

• Are any significant chunks of code duplicated? This will be checked manually as well as with https://github.com/danielstjules/jsinspect

jsinspect (with default config) identified 42 items. Some of these look like they warrant closer inspection. E.g. ItemRateNode contains duplication that should be factored out in options to NumberPicker.

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No plans to address this in old implementation. New implementation will be tested for duplication as part of standard checklist. Closing as "will not fix".