Add endpoint for a group's vulnerable projects in the API

What does this MR do?

• Adds Groups::Security::ProjectsController with an #index endpoint that responds with JSON entities for each of the group's projects that has at least one undismissed vulnerability
• Adds Group#vulnerable_projects which returns a list of projects in the group that have at least one undismissed vulnerability
• Adds a method to Project for each vulnerability severity level that returns the count of undismissed vulnerabilities of that severity that have been reported on the project

https://gitlab.com/gitlab-org/gitlab-ee/issues/11190

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