Add DifferentialDiffQuery and change most callsites

Summary:
Ref T603. This introduces a policy-aware DifferentialDiffQuery and converts most callsites.

I've left unusual callsites (mostly: hard to get the viewer, unusual query, queries related to active diffs) alone for now, so this isn't exhaustive but hits 60-80% of sites.

Test Plan: Created diff; created revision; viewed diffs and revisions; made additional conduit calls.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D6338