room-history: Show read receipts on events

I mostly wanted to check that this feature of the SDK works, as it is a prerequisite to collect read receipts for working on Messages are not marked as read (#1057 - closed).

This is Show other users' read receipts (#115 - closed) but I don't want to close it right away, because it can be improved UI/UX-wise (animations, list the users). But since this is useful enough, I thought it deserved to be added to the codebase in its current state.