[FE] Instance Statistics MVP: object counts

Overview

In &4147, we describe the need for an instance-level overview of instance activity.

backend issue: #233853 (closed)

Proposal

Introduce the Instance Statistics page in the admin panel.

• Show counts for users, projects, groups, issues, MRs, and pipelines. These counts should include the total number of these objects on the instance.
• Query and store these counts daily for performance. Show when the data was pulled.