Add support for partially uploaded files

Summary:
Ref T7149. This flags allocated but incomplete files and doesn't explode when trying to download them.

Files are marked complete when the last chunk is uploaded.

I added a key on <authorPHID, isPartial> so we can show you a list of partially uploaded files and prompt you to resume them at some point down the road.

Test Plan: Massaged debugging settings and uploaded README.md very slowly in 32b chunks. Saw the file lose its "Partial" flag when the last chunk finished.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: joshuaspence, epriestley

Maniphest Tasks: T7149

Differential Revision: https://secure.phabricator.com/D12063