Group Admin access pages under an Access nav category

What does this MR do and why?

Adds an Access category to the Admin Area sidebar, directly below Instance overview, and moves Credentials (Ultimate), Deploy keys, and Certificate authorities under it. The three standalone menus are removed. Follows the design in #628272 (closed).

Certificate authorities keeps its InstanceSshCertificate.available? guard at the item level, so it is still hidden on GitLab.com and when instance_ssh_certificates is disabled. Push rules and Geo keep their current position.

References

• Issue #628272 (closed)
• Parent: #611319 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Sign in as an admin with admin mode enabled and open http://gdk.test:3000/admin.
2. Confirm Access appears directly below Instance overview with a shield icon and contains Deploy keys (and Credentials on an Ultimate license).
3. In rails c, run Feature.enable(:instance_ssh_certificates) and reload. Certificate authorities appears under Access after Deploy keys.
4. Run Feature.disable(:instance_ssh_certificates) and reload. Certificate authorities is gone and Access still renders.
5. Open each page and confirm the Access category is expanded and the item is highlighted.

MR acceptance checklist

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