Partially improve threading UI for adjacent inline comments

Summary:
Ref T10563. This isn't a complete fix, but should make viewing complex inline threads a little more manageable.

This just tries to put stuff in thread order instead of in pure chronological order. We can likely improve the display treatment -- this is a pretty minimal approach, but should improve clarity.

Test Plan:
T10563 has a "before" shot. Here's the "after":

This makes it a bit easier to follow the conversations.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10563

Differential Revision: https://secure.phabricator.com/D15459