Push relevant users if there's a knock on a room (MSC4506)

[ara4n]

Implement MSC4506

Adds the recipient_permission push rule condition; matches if and only if the user the rules are being evaluated for has a power level >= that required to perform the power-levels action named by key.

Uses it in a new default override rule .org.matrix.msc4506.rule.knock (inserted before .m.rule.member_event, which otherwise suppresses all member events) so that members who can invite (i.e. accept the knock) are pushed when someone knocks.

Gated behind experimental_features.msc4506_enabled.

Pull Request Checklist

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