Event requirement calculation is suboptimal

The `prefetch_related` call removes any need for the explicit call to `all`. The current method is suboptimal since two queries are performed: the `all` and the `prefetch_related`.

https://github.com/compserv/hknweb/blob/master/hknweb/candidate/views/officer_portal.py#L186

Also highkey, this entire section could probably be stuck into a chain of `values` into a `count` aggregation. 