check_event_for_spam doesn't get called for new local member events

This issue has been migrated from [#10641](https://github.com/matrix-org/synapse/issues/10641).

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Currently `check_event_for_spam` only gets called for local events in `create_and_send_nonmember_event`, meaning it doesn't get called for new `m.room.member` events, which looks like a thinko. Maybe it should instead be called in `create_new_client_event` which is where we also call `check_event_allowed`, or in `handle_new_client_event`.