Add navigation menu and page for Security inventory

Related to #516875 (closed)

What does this MR do and why?

This MRs adds a new page and navigation changes related to "Security Inventory" feature.

1. Add navigation menu item Security inventory under Secure section
2. Add new route /security/inventory under group level
   • example: http://gitlab.com/groups/group1/-/security/inventory
3. Bootstrap a new vue app and render page heading / description

Render 1. and 2. only when the feature flag is enabledd : security_inventory_dashboard

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable FF: Feature.enable(:security_inventory_dashboard)
2. Visit the inventory page: example (http://gdk.test:3000/groups/flightjs/-/security/inventory)