Add user cohorts table to admin area

What does this MR do?

Adds a table of user cohorts.

Are there points in the code the reviewer needs to double check?

I don't really like the service, but doing the gap-filling and running sums in SQL that worked on both DBs we support really sucked 😞

Why was this MR needed?

To provide an incentive to enable the usage ping! And to let us see the data for ourselves 🙂

Screenshots (if relevant)

When usage ping is disabled:

When usage ping is enabled:

And further down:

What's left?

• Add documentation
• Once https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1377 is merged, switch to using last activity date.
• ???

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/29551.