Implement tools to upload external data to db & access it via scatterplot app

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• [ ] Given data by sub jurisdiction (e.g., census data), upsert data into ExternalData and ExternalDataSet tables.
• [ ] Associate a record in ExternalDataSet to a particular Election
• [ ] Given an election-jurisdiction pair, find -- in order -- all data categories available for major sub jurisdiction (e.g., Population by age, Population by race)
• [ ] Given an e-j pair and a data category, find -- in order -- all available counts (labeled, e.g., by '30 to 49'), along with short source for attribution -- counts only for major subdivision, though.
• [ ] Given e-j pair, data category and count label, get vector of counts (for analysis, display, etc.)

How to handle data missing from only some subdivisions? One option: just ignore it....

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Columns: Election_Id, Jurisdiction_Id, SubJurisdiction_Id, ShortDataSource, Category, Label, Value, InCategoryOrder, DataYear ?