Add excel export to Maniphest

Summary:
Allow Maniphest result sets to be exported to Excel.

Spreadsheet_Excel_Writer is awful but comparatively easy to get working. There's
also a "PHPExcel" package but it has some autoload conflicts right now and this
seems good-enough.

Test Plan: Exported a bunch of tasks to Excel.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T923

Differential Revision: https://secure.phabricator.com/D1721