Collapse previous/forward by default, click to expand.

Summary: This is a lighter UI treatment for previous/forward comments, where the comment is just hidden behind a click. This mayyy be too hard to discover, but I'd rather wait and make it more obvious if actually needed. Once you understand the interaction, the cleaner UI is preferable. Ref T7447

Test Plan:
Test a lot of previous and forward comments.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7447

Differential Revision: https://secure.phabricator.com/D12627