LP#1351355: Switch to XLSX format for Excel report output

LP#1351355: Switch to XLSX format for Excel report output

We've always used Spreadsheet::WriteExcel::Big to generate Excel output
from reports. That module has a limitation of 64k rows, imposed by the
binary Excel format. Recently, the Excel::Writer::XLSX module has become
feature-complete enough that we can use that instead, and the XML format
files that it writes are now supported by the majority of installed
versions of Excel (and OpenOffice) -- the format arrived with Excel 2007,
released in 2006 (8 years ago). So, let's switch to that!

Signed-off-by: Mike Rylander <mrylander@gmail.com>
Signed-off-by: Ben Shum <bshum@biblio.org>