Implement support for creating and updating fragments from ZIPs

Summary:
This implements support for creating and updating fragments from ZIP files. It allows you to upload a ZIP via the Files application, create a fragment from it, and have it recursively imported into Phragment. Updating that folder with another ZIP will recursively create, update and delete files as appropriate.

The logic for creating and updating fragments from files has also been centralized into the PhragmentFragment class. Directories are also now supported; a directory fragment is simply a fragment that has no patches; thus a directory fragment can be converted to a file fragment by uploading a first patch for it.

Test Plan: Uploaded ZIP files through the interface and saw all of the fragments get created and updated as expected.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T4205

Differential Revision: https://secure.phabricator.com/D7729