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

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#### 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.

The issue names no files, tests, or entry points; start by tracing where Mjolnir handles a list removing a user's ban and determines whether other lists still ban that user. Done means notifying moderators when no standing list ban remains while leaving existing room bans unchanged. The quick action versus command choice is still open and needs maintainer direction.

Written by the indexing model from the issue text.