Can respond to friendship requests from memberships bar

lib/lists/memberships_list.dart

@@ -30,4 +30,8 @@ class MembershipList extends AbstractList<Membership> {
   Set<int> get groupsId => where((f) => f.type == MembershipType.GROUP)
       .map((f) => f.groupID)
       .toSet();
+
+  /// Remove a friend membership from the list
+  void removeFriend(int friendID) => remove(firstWhere(
+      (f) => f.type == MembershipType.FRIEND && f.friend.id == friendID));
 }