Render "Show Context" blocks in unified view

Summary:
Ref T2009. This basically copy/pastes them for now. Plans is:

• Make this actually work all the way.
• Add test coverage after D11970.
• Move 2-up here after test coverage.

Clicking the links does not work yet, because they use the 2-up renderer. I'll fix this in the next diff.

Test Plan: Viewed diffs in unified, saw links to show more.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: chad, epriestley

Maniphest Tasks: T2009

Differential Revision: https://secure.phabricator.com/D11976