Add avatar description

closes #175

Author: Brutus5000

api/src/main/java/com/faforever/commons/api/dto/Avatar.java

@@ -17,6 +17,7 @@ public class Avatar extends AbstractEntity<Avatar> {
   @ToString.Include
   private String url;
   private String tooltip;
+  private String description;
   @Relationship("assignments")
   @JsonIgnore
   private List<AvatarAssignment> assignments;