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 whether or not it exists. I'll bring that over in a future change.

Test Plan: Exported users as Excel, opened them up, got a sensible-looking Excel sheet.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13049

Differential Revision: https://secure.phabricator.com/D18955