CRM: Excel export: error messages when opening xls files

[nbarnwell]

Steps to replicate

1. Arrange test data to have Campaign with >1000 members included.
2. Commit Campaign.
3. Click "Download Mailing List".
4. Choose download location for xls file.
5. Open xls file in Excel 2003 (or, I suspect, earlier versions).

Expected Result

Spreadsheet opens with no formatting (except bold applied to header row) and no messages.

Actual Result

Spreadsheet opens, but Excel displays a warning message:

Too many different cell formats

Also, the background colour of all the cells is set to white. I think this is a side-effect of Excel being upset, rather than anythin in the GiveCRM code. If we fix the formatting problem, I believe the cell background formatting will disappear. Clicking okay displays data and a second messagebox:

"Excel encountered an error and had to remove some formatting to avoid corrupting the workbook. Please check your formatting carefully."

Additional Notes

Although this isn't a problem with newer versions of Excel, I suspect the kind of charities we're targeting with GiveCRM might well have this issue, so it's not something to ignore. I believe this is because the Excel export tool we're using is applying formatting to each cell individually, rather than to an Excel Range of cells.