Adds the page for instance lvl security dashboard

What does this MR do?

This MR sets the groundwork for #6953 (closed)

It adds the route /-/security, along with the associated controller, template, and JS required to get started.

Since it currently renders and empty dashboard with no endpoints, the whole thing is hidden behind the security_dashboard feature flag. If that flag is enabled, it will render a completely empty dashboard.

This MR is completely ~backstage and simply helps bootstrap the setup of this new dashboard.

See also the CE port.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content