In 9969b01c5f we stopped updating the sublist whenever we felt like it, which indirectly froze message previews for room tiles (badges, unread state, etc were unaffected because that is managed by a different store). To fix this, we simply have to listen for changes and perform an update.
This means we're abusing the AsyncStoreWithClient to get access to a lifecycle, but overall that seems like a minor crime compared to the time spend abusing the store's state as a map.
With thousands of rooms shown, we can save on average 743ms per preview. The new preview time is 0.12ms on average.
This reverts earlier changes made to textForEvent as they are no longer needed.
This also implements an entire tree of textForEvent-like behaviour as the previews need to be different, which is easiest done with its own stack.
