fix: Set room to read when last message is an update message

[SystemKeeper]

• Related iPad view left sidebar does not "cheat" unread counter anymore #1123

We set the room to read, when we enter the room or receive new messages. Currently we enforce that the last message needs to be a non-update message, so when the last message is a reaction or edit, we will not update the room. This PR changes the logic a bit, so that we

1. Always set the room to read and only update the last message in case there's one
2. Check if we can find a non-update message, instead of relying on the last message

It's still a bit problematic if the server is very slow and we receive "old" data, but it should improve things.