Reduce derps in timeline event group rendering

Summary:
Ref T4266. This possibly moves us towards getting reasonable timeline grouping:

• Always sort icon stories to the top.
• Render one timestamp for the whole group, using the earliest tranaction date.
• Move any "Edit", "Edited", or "Preview" links to the top.
• Rendering just one timestamp implicitly fixes the JS issues.
• For stories without an icon, indent them if any member of the group has an icon.

Test Plan: See screenshots.

Reviewers: chad, wotte

Reviewed By: chad

CC: aran

Maniphest Tasks: T4266

Differential Revision: https://secure.phabricator.com/D7842