Add breadcrumbs to admin role promotions

What does this MR do and why?

It adds a breadcrumb and proper page title to the admin role promotions page.

Note: the MR doesn't have a changelog since the changes are behind a feature flag.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

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Before	After

How to set up and validate locally

1. Have an Ultimate License, and simulate Ultimate Self Managed setup (export GITLAB_SIMULATE_SAAS=0 gdk restart)
2. Enable the Setting ApplicationSetting.first.update!(enable_member_promotion_management: true)
3. Enable feature flag Feature.enable(:member_promotion_management)
4. Create promotion requests:
   1. visit a group as an owner (e.g. http://gdk.test:3000/groups/gitlab-org/-/group_members)
   2. promote a non-billable GUEST member to a DEVELOPER role (this member should be non-billable on the whole instance, i.e. have a highest role as guest, see !149094 (comment 1869616221) for details)
   3. you should see a banner saying "Role change request was sent to the administrator." — this means a promotion request has been created
5. As root go to Admin area > Users > Promotion Requests (e.g. http://gdk.test:3000/admin/role_promotion_requests)