Implement snapshots in Phragment

Summary:
Ref T4212.  This implements snapshots in Phragment, which allows you to take a snapshot of a fragment at a given point in time, and download a ZIP of the snapshot as it was in this state.

There's also functionality for deleting and promoting snapshots.  You can promote a snapshot to either the latest version or any other snapshot of the fragment.

Test Plan: Clicked around, took some snapshots, promoted them to different points and deleted snapshots.  Also downloaded ZIPs of the snapshots and saw the right versions coming through for all the files downloaded.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T4205, T4212

Differential Revision: https://secure.phabricator.com/D7741