Summary:
This adds support for different export formats to Excel
via a drop-down on the Export page as per the discussion
in D5443.
Test Plan:
Export some things from Maniphest. Do a simple implementation
of ManiphestExcelFormat just for testing and make sure that
it appears in the list after you run `arc liberate`.
Reviewers: epriestley
Reviewed By: epriestley
CC: aran, Korvin
Maniphest Tasks: T2575
Differential Revision: https://secure.phabricator.com/D5642
Conflicts:
src/applications/maniphest/controller/ManiphestExportController.php
