Add vulnerability findings data to instance security dashboard

What does this MR do?

Adds endpoints for fetching vulnerability findings for the instance security dashboard.
Reuses the controller concerns backing similar endpoints for the group and project security dashboards.

• Adds Security::VulnerabilityFindingsController and corresponding routes
• Adds SecurityHelper
• Adds an ApplicationInstance PORO model to represent the data side of the dashboard

Screenshots

Screen_Recording_2019-12-12_at_5.54.40_PM

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

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

Closes #33899 (closed)