Support XEP-0333 (chat markers) in MUCs

[aereaux]

It would be great if we could see who has read our messages inside MUCs like we can see if our 1:1 conversation partner has read our recent messages.

Quick mock-up, please let me know if you have any feedback:

The avatar images could have tooltips with the name in case you don't recognize it.

I was going to take a look at the code to see what would need to be implemented for this, but I wasn't entirely sure where to start for the MUC read markers backend, if they were already recorded or not.

[licaon-kter]

Duplicate of #240 ?

[aereaux]

I don't think it's a duplicate as I understand that issue. That seems to be where some chat markers don't get sent from dino or received by conversations. I'm proposing here to display chat markers from other users' clients in MUCs within dino.