Add project security dashboard vulnerabilities endpoints

What does this MR do?

This MR adds endpoints for fetching vulnerabilities for the project security dashboard. It:

• Includes Vulnerable in Project
• Adds routes for fetching the vulnerabilities
• Adds Projects::Security::VulnerabilitiesController, which includes VulnerabilitiesActions and SecurityDashboardPermissions

It is the final MR of 5 for adding these endpoints (yay!), and fixes https://gitlab.com/gitlab-org/gitlab-ee/issues/12381 and https://gitlab.com/gitlab-org/gitlab-ee/issues/12244

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team