Draft: Prepare reading admin general settings as custom permission

What does this MR do and why?

It adds preparation for reading general settings custom admin permission.

The permission usage implementation (controllers etc.) will be added as another MR to keep the scope small as it requires multiple changes in view and vue app.

References

Screenshots or screen recordings

How to set up and validate locally

1. Enable feature flag Feature.enable(:custom_admin_roles)
2. Go to Admin -> Settings -> Roles and Permission, http://gdk.test:3000/admin/application_settings/roles_and_permissions
3. Start creating a new admin role, make sure you can create a new role with Read admin general settings

MR acceptance checklist

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

Related to #527112