Fix suboptimal DB queries

[nkorobkov]

Problem
Some pages load very slow
I suspect that code has places where unnecessary joins are executed during page rendering.

What to do
Look through the code and find all suspicious places.
Fix all problems with Django object annotations
https://docs.djangoproject.com/en/2.2/topics/db/aggregation/

Also:
https://medium.com/@hansonkd/the-dramatic-benefits-of-django-subqueries-and-annotations-4195e0dafb16

Criteria
There are no DB queries in a loop and no extra joins in the code.

[nkorobkov]

https://docs.djangoproject.com/en/3.0/ref/models/querysets/#django.db.models.query.QuerySet.select_related