R1:df053abd6ec1

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.