Performance optimization in Invoice admin page

The total amount is a useful property to show in the overall view, as it allows one to check from the bank statement which invoice it is. However, loading this total amount is also very slow, as it requires checking every event's player count.

This merge request makes the invoice more snappy by doing some prefetch (from 370 queries to 224 on the SUL backup) and caching the total amount as displayed on the admin interface. We only cache on the admin page so that the PDFs are always 100% correct. This should be OK as PDFs are not very often downloaded all at once.