Clean up Cohorts

Description

User cohorts is a mess. Let's clean it up.

Proposal

1. Remove inactive users, this is confusing
2. Cohort total should just be called New users (and we should verify this is what it represents)
3. The first month we list should be Month 1. Everyone will have activity in Month 0, since we count any kind of activity.
4. Remove the usage ping data from this page. The usage ping itself should just be a page with just that data, opened in a new tab. It should be linked from the actual setting.

Links / references

changed milestone to %Next 2-3 months in gitlab-foss

added Deliverable in GitLab FOSS ~874211 labels

moved from gitlab-foss#34558 (moved)