Add promotion callout for security training feature to vulnerability report (UI only)

Add a promotion callout on the security dashboard. The CTA button is a link to the security training configuration page.

Note: This only adds the related UI elements and none of the logic to store dismissal states, etc.

Implementation Plan

• Add banner to the security dashboard
• Add feature flag to the related controller
• Pass the CTA link from HAML to JS
• Inject link and use it within the banner component
• Write tests