Display user and group icons on the workflow roadmap instead of "user:" and "group:"

On the list of reviewers, it currently display "user:sguitter" or "group:members". It should display the user and group icons.

The list of reviewers is not clear for the user: it should be clear that the goal is to show to the user if the task was assigned to him only, him+ other members, him as part of a group etc.
We should add a label (or any other idea?) to clarify that.