Generate exact counts of user/projects/groups in /admin

This relates to https://gitlab.com/gitlab-org/gitlab-ce/issues/52096. The /admin page relies on table estimates that can vary anywhere from 1-15% depending on the database row estimates.

This degrades trust in the product, as it's better to be slow but accurate. We should either:

1. Consider doing a background update of the counts periodically whenever an admin visits /admin
2. Raise the thresholds where we do an exact count