We do this by a) not pulling out all membership events, and b) batch
inserting bans.
One blocking concern is that this bypasses the `update_membership`
function, which otherwise all other membership events go via. In this
case it's fine (having audited what it is doing), but I'm hesitant to
set the precedent of bypassing it, given it has a lot of logic in there.
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Co-authored-by: Eric Eastwood <erice@element.io>
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