Update some inline comment logic to use more modern "Viewer"-oriented calls/variables

Summary:
Ref T13195. Ref T8573. The inline comment controllers currently use outdated $user = $this->getRequest()->getUser() calls.

Instead, use $viewer = $this->getViewer().

This is just a small consistency update with no behavioral changes.

Test Plan: Viewed and added inlines in Differential and Diffusion.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13195, T8573

Differential Revision: https://secure.phabricator.com/D19633