Adds a confidence filter to the GSD

What does this MR do?

Adds a confidence filter to the Group Security Dashboard. It behaves in much the same way as the other filters in the dashboard. This came up in one of the workshops at Contribute.

This MR adds both the front and back end components.

CE port for that one line in the docs https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/28410

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