Creates a standalone vulnerability page

What does this MR do?

This MR adds in the standalone vulnerability page template. The data is currently all in HAML and is behind the first_class_vulnerabilities feature flag. For information on how to toggle this feature flag in development, see the docs.

In order to do this, I've had to change the way rails thinks about the current dashboard. It's no-longer a standalone page with a show template, but is now the index page for all vulnerabilities. It makes no user-facing difference, but it has move a few templates about in the code.

Walkthrough

first_class_page

create_standalone

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

/label ~backstage

touches #36739 (closed)