Add SaaS Usage and User dashboard page

What does this MR do and why?

Initialize Usage and User dashboard pages in SaaS groups

References

https://gitlab.com/gitlab-org/gitlab/-/issues/569418+

Screenshots or screen recordings

User dashboard	Usage dashboard

How to set up and validate locally

• Ensure you're running GitLab as SaaS (e.g. export GITLAB_SIMULATE_SAAS=1 gdk restart)
• Ensure you've a subscription added to your group
• Enable billing_usage_dev feature flag (e.g. http://gdk.test:3000/rails/features/usage_billing_dev)
• Navigate to the group > Settings > GitLab Duo (e.g. http://gdk.test:3000/groups/gitlab-org/-/settings/gitlab_duo)
• Append /usage to the URL (e.g. http://gdk.test:3000/groups/gitlab-org/-/settings/gitlab_duo/usage) to test the usage dashboard
• Append /usage/users/12345 to the URL (e.g. http://gdk.test:3000/groups/gitlab-org/-/settings/gitlab_duo/usage/users/12345) to test the user dashboard

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 #569418