Align group and project level security dashboard UX

What does this MR do?

The purpose of this MR is to make the UI and UX more consistent between the group security dashboard and the project security dashboard.

This is done by:

• Using the SecurityDashboardApp component in projects security page, instead of the SplitSecurityReportApp component
• Modifying the project_security_dashboard_config helper so it returns the data that's required for the dashboard to work properly
• Adding the notion of "locked filters" to the SecurityDashboardApp, which basically lets us preset the projects filter and hide it from the UI
• Cleanup any code that might become useless after these changes

What does it look like?

Here's what the project security dashboard currently looks like:

Here's what it looks like after these changes:

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
  • Follow up MR created: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/28681
• Code review guidelines
• Style 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
  • Note: I was unable to test the dashboard on IE11, the main container remained blank both in the Group Security Dashboard and the Project Security Dashboard. I'll do more testing and follow up in a separate issue if I can confirm there's a problem here.
    • Follow up MRs:
      • https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/13319
      • https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/28732

Closes #7710 (closed)