create_extra_permissions tries to create a permission eight times for every model every time ./manage.py migrate is used. In a project with a big number of models this process can take more than a minute.

This update minimizes the number of SQL queries by: * selecting all needed content types at once; * selecting information about existing relevant permissions to a set that is used to check for existence of a permission before trying to create it; * creating permissions using bulk_create.

./manage.py migrate runs 9 times faster with this change in my case when no new migrations are applied.