style: Check power levels before attempting to send extremity squashes

Solves a problem where the console screams in agony when local users can't send dummy events

src/service/rooms/event_handler/handle_incoming_pdu.rs

@@ -271,8 +271,19 @@ pub async fn handle_incoming_pdu<'a>(
 			"Attempting to squash extremities after upgrading pdu"
 		);
 		// Try to send a dummy event to squash extremities. See issue #1844
+		let power_levels = self
+			.services
+			.state_accessor
+			.get_room_power_levels(room_id)
+			.await;
 		let mut local_users = self.services.state_cache.local_users_in_room(room_id);
 		while let Some(user_id) = local_users.next().await {
+			if power_levels.for_user(&user_id)
+				< power_levels.for_message("org.matrix.dummy_event".into())
+			{
+				trace!(%user_id, "user does not have power level to send dummy event, skipping");
+				continue;
+			}
 			let state_lock = self.services.state.mutex.lock(room_id).await;
 			if self
 				.services