Update Standalone Vulnerabilities page to be a single Vue app

NOTE: This issue is blocked by #215069 (closed). The description below is assuming that #215069 (closed) has been completed.

Currently, the standalone vulnerabilities page is 3 separate apps:

It should be combined into one root Vue app with the header, details, and footer components.

Acceptance Criteria

• There is only one root app for the standalone vulnerabilities page.
• The root app contains the header, details, and footer components.
• The vulnerabilities event bus is removed.
• The vulnerabilities event bus logic is moved into the root app. Changing the status of a vulnerability using the dropdown in the header should reload the discussions in the footer:

Developer notes

File to touch	Explanation
ee/app/assets/javascripts/pages/projects/security/vulnerabilities/show/index.js	Contains setup code for the 3 apps, needs to be combined into one setup for the new Vue root app.
ee/app/views/projects/security/vulnerabilities/show.html.haml	Contains the 3 placeholder divs, needs to be combined into one placeholder div used by the setup code.
ee/app/assets/javascripts/vulnerabilities/components/vulnerabilities_event_bus.js	Event bus that triggers the footer to reload its discussions when the vulnerability state dropdown in the header is changed. This bus should be removed and the logic moved into the new Vue root app.