Admin dashboard group approximate estimate will never be accurate for large instances

For performance reasons we use an approximate value for model counts for the admin dashboard. https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/gitlab/database/count/tablesample_count_strategy.rb

This is accurate for small instances (less than 10,000 records) and is also accurate enough for certain models passed that point. However, some things like Group will never be accurate for large instances. This is because a Group is actually a subset of Namespace. The approximate count will get all types of Namespaces rather than just Groups.

A customer ran into this in https://gitlab.zendesk.com/agent/tickets/111450, but the problem for them is actually solved by https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22650. In talking through this with @yorickpeterse I realize the problem remains for larger instances.

cc/ @stanhu