Default values will be 1 room per minute, with a burst count of 10.
It's hard to imagine most users will be affected by this default rate,
but it's intentionally non-invasive in case of bots or other users that
need to create rooms at a large rate.
Server admins might want to down-tune this on their deployments.
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Signed-off-by: Olivier 'reivilibre <oliverw@matrix.org>