FIX: Give the topic timeline unique keys for state (PR #14305)

We’ve observed an error where the back button is displayed improperly in the topic timeline. It’s unfortunately been hard to reproduce but we suspect it’s related to leftover state when re-rendering.

This fix optimistically tries to fix the error by introducing the topic’s id to the unique key the widgets use for state. We can deploy this and keep an eye out for the bug in the future.