Support requiring multiple confirmations before a ban is issued

[turt2live]

This takes two forms:

1. Literally the entity has to match N rules before they'll be banned, across any number of lists.
2. The entity must match N lists before they'll be banned. The lists valid for counting N might be specified by the moderation team.

Multiple confirmations are useful for communities who want to subscribe to another community's lists, but don't want to subject control of their rooms to that community. A good example of this is any community which wants to use the matrix.org homeserver lists, but isn't willing to let us ban whoever we want in their rooms.