Add approve and reject actions to admin pending promotions management

What does this MR do and why?

Adds Approve and Reject actions to the admin promotion management panel.

Part of FE: Add a “Pending promotion” tab to the admin ... (#433177 - closed)

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.

Screenshots or screen recordings

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state	Before	After
loaded
approved	N/A
error while approving	N/A

How to set up and validate locally

1. Have an Ultimate License, and simulate Ultimate Self Managed setup (export GITLAB_SIMULATE_SAAS=0)
2. Enable Setting setting = ApplicationSetting.first; setting.enable_member_promotion_management=true; setting.save!
3. Enable FF Feature.enable(:member_promotion_management)
4. Create promotion requests:
   1. Visit any Group as an owner of that group (e.g. http://127.0.0.1:3000/groups/gitlab-org/-/group_members)
   2. try promoting 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 an admin open Admin area > Users > Promotion Requests (e.g. http://localhost:3000/admin/role_promotion_requests)
6. Try Approving or rejecting some of the requests (blocking network requests is possible now)

Currently its easy to restore those requests: run rails c + Members::MemberApproval.update_all(status: :pending)

Storybook

This MR also adds a Storybook story for this app's top component, start the storybook with npm run storybook:start and open http://localhost:9002/?path=/story/ee-admin-role-promotion-requests-app--default