Fix the "Load More" button always appearing on the circle timeline

Here we copy the technique used for the similar button in the regular (non-circle) TimelineView. The trick is to handle errors thrown in the pagination function. 🤦

Because we're also relying on the CircleSpace to republish Combine updates from its child Rooms, we also (attempt to) fix a concurrency warning by receiving the updates from child rooms on the main thread.

Closes #317 (closed) and #329 (closed)