Implement "Download ZIP" controller for Phragment
Summary: Depends on D7727. This adds support for downloading a fragment and all it's children as a ZIP file. Fragments that have children automatically become directories in the ZIP file.
Test Plan: Downloaded a fragment as a ZIP and was able to extract the contents successfully.
Reviewers: epriestley, Blessed Reviewers
Reviewed By: epriestley
CC: Korvin, epriestley, aran
Maniphest Tasks: T4205
Differential Revision: https://secure.phabricator.com/D7728