Link to undisplayed inline comments

Summary:
We currently don't link to comments which aren't visible. Link to the
appropriate diff in a new window, indicating where the comment lives.

Test Plan: Clicked visible, not-so-visible comments.

Reviewers: btrahan, jungejason, davidreuss

Reviewed By: btrahan

CC: aran, btrahan, epriestley

Maniphest Tasks: T555, T449

Differential Revision: https://secure.phabricator.com/D1333