Add "security" top-level item to group sidebar

What does this MR do?

It adds a new tab, "Security" to the navigation-sidebar on the group section, which links to the "Security Dashboard".

Secondly it removes the previous link to the security-dashboard, which was sitting underneath the project tab.

Note: Ports 30604 to EE

MR that updates related documentation can be found here https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31046

Before

After

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.