Replace statistics.file with statistics.spreadsheet_column

[PhantomWatson]

Currently, a statistic is linked to a spreadsheet column by a chain of associations:

statistics has one metrics hasMany* spreadsheet_columns_metrics
And a statistic can be matched to a single spreadsheet column when statistics.file matches spreadsheet_columns_metrics.filename.

• This association is never used, so it's not actually defined in MetricsTable.

It would be simpler and would reduce the size of the statistics table to replace statistics.file with statistics.spreadsheet_column_id. Incidentally, it now seems preferable to rename spreadsheet_columns_metrics to just spreadsheet_columns.

Steps

• Note the size of the statistics table
• Rename spreadsheet_columns_metrics to just spreadsheet_columns and update class names in code
• Create a statistics.spreadsheet_column_id field
• Write a populate-ss-column-ids script to write to this field for every statistic record
• Have this field be written to when import-stats creates or updates a record in statistics
• Delete the statistics.file column and remove references from code
• Note the new size of the statistics table and by what percent its size was reduced