Be able to see the assignees in the issues list on Epics

Description

(Include problem, use cases, benefits, and/or goals)

Now I have to go to the issue page to see if a person take over the issue while browsing the issues list on epics page. If I can see the assignee in the issues list on epics as well, I can pick up an issue more quickly.

Proposal

Having an assignee avatar in the issues list.

cc/ @dimitrieh