Create new feature flag for admin settings

Problem to solve

The admin settings is currently behind the cube_api_proxy feature flag. The problem with this approach is that this flag is also used for enabling the Product Analytics API endpoints.

Proposal

Add a new product_analytics_admin_settings feature flag to separate the admin settings from the API.

Implementation plan

1. Create a new product_analytics_admin_settings feature flag
2. Update ee/app/views/admin/application_settings/_product_analytics.html.haml#L1 to use the new flag
3. Update specs