Large contests in the task archive load slowly

[kyepskee]

Opening the page for a specific contest (e.g https://szkopul.edu.pl/task_archive/oi/) takes a long time. It clearly shouldn't, because it's almost a static page with around a 100 list of entries of size on the order of 100. Improvements could probably be made by optimizing database queries inside the task_archive_tag_view and the sorting it requires.