Refactor ADD_FILTER in List active reducer to use _.unionWith
Refactor ADD_FILTER in List active reducer to use _.unionWith

Before, this case was unreadable, but by leveraging lodashs unionWith we
can remove all of that illegible code and make it work.

Compared using the newly added tests, it works exactly as before. :+1:

creynders/keystone,creynders/keystone