Add search settings to admin settings

• WIP Full Context MR
• Refactoring: Move search settings mount function to the search_settings directory (merged)
• Fix flashy search box when loading a settings page (merged)
• Add enable_search_settings_helper (merged)
• Separate sections in user settings pages layout (merged)
• Add search to user settings
• Document search settings
• Settings search bar to remember expansion state
• Add search to admin settings

What does this MR do?

Screenshots (strongly suggested)

• With search_settings_in_page enabled
• In Admin > Settings

General	Advanced Search	Integrations	Repository

Templates	CI/CD	Reporting	Metrics

Network	Preferences	Does not show outside Settings 🎉

Does this MR meet the acceptance criteria?

Conformity

• [ ] Changelog entry Not needed since added behind disabled-by-default feature flag.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content