Allowing hiding the admin perspective through a server flag

Long term, we want to migrate the admin perspective to its own dynamic plugin. In the short term, we can provide a flag to hide the perspective so that console can be used as a framework for building other experiences like HAC.

Acceptance Criteria

• The backend has a flag that will disable the admin perspective.
• When the flag is set, the admin perspective and its nav items are hidden.
• If there is only one perspective, hide the perspective switcher.

Non-goals

This story does not cover moving any of the admin console code out of the public package or migrating it to a dynamic plugin. That will be covered by separate stories in epic CONSOLE-2943.