a11y: use role="button" for all avatars

[web-padawan]

From #10 (comment)

In my understanding it's more like a button / a group of buttons, especially if you are going to add a clicklistener or space-listener to it for interaction. (For example, to show more informations about the selected user in a dialog)

Currently, role="image" is used for avatars, except the +N avatar, which has role="button". This is confusing.
We should consider using role="button" for all the avatars consistently.