Vulnerability Report in instance security dashboard

Navigational/page-structure changes to instance-level security dashboard.

See Design Issue and parent Epic for details.

Implementation Plan

• frontend create instance-level security chart component and update CSS for vulnerability severity widget
• frontend use the instance-level security chart component for the security dashboard page, create a Vulnerability Report menu item and create a new vulnerabilities route to serve up the instance-level security dashboard component. Remove the widgets from the instance-level security dashboard component. Update docs.