Whenever we already deal with a User object, let's use the more specific
method avatar_icon_for_user.

Closes #33124

If we rely on the helper functions in `GitlabRoutingHelper` for `merge_request_path`,
we end up calling a database query to look up the Namespace association for
every merge request since `entity.project.namespace` is called. By reusing the project
defined in the controller, we avoid that problem.

Partial fix to #27387

The issuable reference wasn't correctly rendered on the page

2 counts where rendered in the top bar of the panel