Add admin setting section for non-billable users promotion management

What does this MR do and why?

Adds an admin settings section for Promotion Management for Self Managed instances.

Resolves #452276 (closed)

MR acceptance checklist

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Before	After

How to set up and validate locally

1. Ensure the environment is right:
   1. Simulate a self managed instance (GITLAB_SIMULATE_SAAS=0 in env for gdk)
   2. Purchase an Ultimate licence
   3. enable member_promotion_management feature via rails c and Feature.enable(:member_promotion_management)
2. Go to admin area > settings > general > sign up restrictions
3. Check that the Approve role promotions checkbox is present and preserves it's state when reloaded
4. When enabled — admin users page http://localhost:3000/admin/users should contain "Role promotions" tab