Allow transactions to be grouped in the TimelineView

Summary: TimelineView on Maniphest is often kind of hard to parse because related/simultaneous transactions aren't visually grouped. Allow grouping. I'm going to clean this up a little bit more.

Test Plan: See screenshot.

Reviewers: chad

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D7107