R1:8bb6e807f097

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…