Instance statistics should be available without usage ping

We added instance statistics in https://gitlab.com/gitlab-org/gitlab-ce/issues/41416, but made it dependent on an instance having usage ping enabled. We should never incentivize turning usage ping off, and it shouldn't be a requirement for showing instance statistics on an instance.

Proposal

If usage statistics are enabled on an instance, present the instance statistics area of the product. Since Cohorts is dependent on usage ping, don't present this as an option in the sidebar.