Make DifferentialRevisionQuery policy-aware

Summary:
Ref T603. Ref T2625. Makes `DifferentialRevisionQuery` do policy checks.

Note that it still uses inefficient offset-based paging, but it's rare to page through revisions. I'll switch to cursor paging in a future diff.

Test Plan: Viewed a bunch of Differential interfaces, home page, etc. This shouldn't actually materially impact anything.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603, T2625

Differential Revision: https://secure.phabricator.com/D6344