Resolve link framing issue for public feed

Summary:

• Use $this->linkTo($phid) to render all links.
• Simplify code.

Test Plan: Public feed renders with 'target="_top"' links. Nonpublic feed
doesn't. Looked at a bunch of feed stories, none seem broken.

Reviewers: btrahan, aran, nh, jungejason, ide

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T453

Differential Revision: https://secure.phabricator.com/D1514