Mjolnir should notify when no more lists mark a user as banned

[ShadowJonathan]

Is your feature request related to a problem? Please describe.

When one or more lists ban a user, it could happen (through social deliberations) that that same user gets removed from all these lists.

Currently, when mjolnir observes the last list removing a user's ban, it does not unban this user from existing rooms.

Describe the solution you'd like.

Mjolnir should not unban these users directly (for a variety of reasons), but it should at least notify that the ban removal was the last remaining one across all lists.

This could be paired with a "quick action" reaction on the same message that a moderator can press to unban the user in question from all affected rooms. Alternatively, the bot could present a command to enter to unban this user in the same message.

The key point here is to not unban a user automatically once it is removed from all lists, but rather notify that it observes no list with standing policy to ban a certain user.