Allow an unban in one list to take precedence over a ban in another

[Cadair]

If you are subscribing to another list (read only) you should be able to have a way to overrule actions taken on that list by adding things to one of your own banlists.

The most logical way to do this to me would to have list have priorities and have unbans in higher priority lists overrule bans in lower priority ones. By default I would suggest that read only lists have lower priorities than writeable ones so this functionality works without extra config.