R1:040927959546

Support Excel as a data export format

Summary:
Depends on D18954. Ref T13049. This brings over the existing Maniphest Excel export pipeline in a generic way.

The `<Type>ExportField` classes know directly that `PHPExcel` exists, which is a little sketchy, but writing an Excel indirection layer sounds like a lot of work and I don't anticipate us changing Excel backends anytime soon, so trying to abstract this feels YAGNI.

This doesn't bring over the install instructions for PHPExcel or the detection of…