Adds the security dashboard link

What does this MR do?

1. Removes redirect from the group security dashboard page.

2. Adds the required before_filters viw application_controller.rb.

3. Add navigation item connected with permission check,

Note: I'm 99% sure I've done the last step wrong. I could do with some guidance on this.

Also Note: This should probably have tests, but again I'm not sure where to put them.

!7971 (closed)

CE MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22549

What are the relevant issue numbers?

#6709 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?