Use spreadsheets/google sheets when importing expense information

[ezwang]

A lot of clubs have their expense calculations in a spreadsheet, and it might be easier for them to import a (excel/csv) spreadsheet instead of manually entering expenses onto the application page.

The major difficulty would be figuring out how to parse the uploaded information. We could either specify a format that the spreadsheet must be in, or attempt to detect relevant fields in the spreadsheet.

We could just require a CSV format (Excel and google sheets both export to CSV) so we don't have to deal with that many formats.