Fixes https://github.com/matrix-org/mjolnir/issues/15
This should be completely safe to deploy, even without a labs flag, because policy rooms (ban lists) are usually only joined by the people who know what they are. This also has no relation to the Mjolnir functionality currently in Riot, though it does make seeing your ban list a bit less awkward.
Ban list rules only show up in ban list rooms and rarely show up in public rooms, hence the somewhat vague messaging. It should be obvious that it's a ban list room due to the surrounding room decoration/purpose.
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