Implement improved rendering for threads in the main timeline

[janogarcia]

Changelog

Root messages should form a standalone group with their respective thread summaries.

This way, we establish a clear relationship between the thread summary and the root message it belongs to. Failure to do it results in an ambiguous relationship that only gets worse when message bubbles are enabled.

Source

→ https://www.figma.com/file/T309ztx0sNyOOK6NKVLHsK/Threads?node-id=3085%3A305958

Snapshot