Use RepositoryQuery along common pathways

Summary: Ref T603. Make common repository queries (in Conduit and DiffusionRequest) policy-aware. These tend to get caugh by something else anyway, but tighten them up.

Test Plan: The conduit change already provided `user` everywhere. I verified that and browsed some pages.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D7060