More optimizations; /results/events

Decreased the number of queries and the total query time from
approx. (170, 2500ms) to approx. (80, 150ms).
More optimizations; /add/review

Same changes as for 47edfbb.
More optimizations; /results

Number of queries decreased from approx. 190 to approx. 30 by using
select_related. This greatly decreases CPU usage.