Add UI for "Group Overview default" preference in User Settings

What does this MR do?

• This MR is about the "Group Overview default" preference (User -> Settings -> Preferences), both database support for it and UI to update it
• It's a part of changes introduced for https://gitlab.com/gitlab-org/gitlab-ee/issues/7048
• Another part is the behavior of the Group Overview according to this setting: !7889 (merged)
• Documentation, changelog and other checklist items related to #7048 (closed) will be enforced in !7889 (merged)

What are the relevant issue numbers?

#7048 (closed)

Corresponding CE MR

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24761

Does this MR meet the acceptance criteria?

• [ ] Changelog entry added, if necessary
• [ ] Documentation created/updated via this MR
• [ ] Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• [ ] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• ~~[ ] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.~~
• 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?
• Security reports checked/validated by reviewer