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