* DEV: Add a dedicated Admin::StaffController base controller
The current parent(Admin:AdminController) for all admin-related controllers
uses a filter that allows only staff(admin, moderator) users.
This refactor makes Admin::AdminController filter for only admins as the name suggests and
introduces a base controller dedicated for staff-related endpoints.
* DEV: Set staff-only controllers parent to Admin::StaffController
Refactor staff-only controllers to inherit newly introduced
Admin::StaffController abstract controller. This conveys the
purpose of the parent controller better unlike the previously used parent
controller.
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