ChangeScreen: Add icon for adding reviewer

Replace 'Add' button with an icon. This is consistent with editing the
topic which is also done by clicking on an icon.

The user_add icon is taken from the silk icons [1].

[1] http://famfamfam.com/lab/icons/silk/

Change-Id: I5af8bb2814437bf77c0eaafc5c9dd131f802e6d4
Signed-off-by: Edwin Kempin <ekempin@google.com>

gerrit-gwtui-common/src/main/java/com/google/gerrit/client/Resources.java

@@ -24,6 +24,9 @@ public interface Resources extends ClientBundle {
   @Source("note_add.png")
   ImageResource addFileComment();
 
+  @Source("user_add.png")
+  ImageResource addUser();
+
   // derived from resultset_next.png
   @Source("resultset_down_gray.png")
   ImageResource arrowDown();