Set Security Dashboard as default view for groups

What does this MR do?

• This MR gives a user the option to display security dashboards for groups by default instead of the subgroup and projects view.
• It's a part of changes introduced for https://gitlab.com/gitlab-org/gitlab-ee/issues/7048
• Another part is the BE and UI for changing the setting "Group Overview default" in User -> Settings -> Preferences: !9332 (merged)

What are the relevant issue numbers?

#7048 (closed)

Corresponding MR in CE

Adds some supporting changes:

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22777

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [ ] Documentation created/updated - a separate MR is created for that: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9906
• 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?