Fix delete trigger (#317)

I've tested this again by deleting my full history and re-syncing, all
good.

atuin-server/migrations/20220421073605_fix_count_trigger_delete.sql

@@ -0,0 +1,35 @@
+-- the old version of this function used NEW in the delete part when it should
+-- use OLD
+
+create or replace function user_history_count()
+returns trigger as 
+$func$
+begin
+	if (TG_OP='INSERT') then
+		update total_history_count_user set total = total + 1 where user_id = new.user_id;
+
+		if not found then
+			insert into total_history_count_user(user_id, total) 
+			values (
+				new.user_id, 
+				(select count(1) from history where user_id = new.user_id)
+			);
+		end if;
+		
+	elsif (TG_OP='DELETE') then
+		update total_history_count_user set total = total - 1 where user_id = old.user_id;
+
+		if not found then
+			insert into total_history_count_user(user_id, total) 
+			values (
+				old.user_id, 
+				(select count(1) from history where user_id = old.user_id)
+			);
+		end if;
+	end if;
+
+	return NEW; -- this is actually ignored for an after trigger, but oh well
+end;
+$func$
+language plpgsql volatile -- pldfplplpflh
+cost 100; -- default value