Give auditors read only access to billings page

What does this MR do and why?

Resolves #372964 (closed).

This creates a read-only view mode for auditors so that they can see the billing page but not make modifications. This is behind the :auditor_billing_page_access feature flag.

Screenshots or screen recordings

	Before	After
Owner view (unchanged)
Auditor view

How to set up and validate locally

• Setup and run CustomersDot
• Within your gitlab terminal window, run export GITLAB_SIMULATE_SAAS=1
• If GDK is already running, restart it
• Run rails c to open the rails console
• Run ::Gitlab::CurrentSettings.update(check_namespace_plan: true) and ::Gitlab::CurrentSettings.update(automatic_purchased_storage_allocation: true)
• Enable :auditor_billing_page_access feature flag

Group Billing Page

• Ensure you have a group where you're an owner, or create one
• Verify that you can still see the correct buttons on groups/<group-name>/-/billings
• Create an auditor user, invite them to the group, then impersonate as that user
• Verify that you cannot see any buttons on the subscription table

Profile Billing Page

• Regression test that all user types are able to access :3000/-/profile/billings

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.