Share code for context link rendering on side-by-side diffs

Summary: Ref T2009. I've clicked these links like 200 times in testing now, so I'm feeling pretty good about them.

Test Plan: Viewed links in side-by-side diff, clicked them.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2009

Differential Revision: https://secure.phabricator.com/D11981