Settings search bar to remember expansion state

• 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?

This MR resolves issue identified in this comment !53581 (comment 503655640) where using the search bar in settings doesn't remember the expanded/collapsed state of settings before the user started a search.

How to test?

• Feature.enable(:search_settings_in_page)
• Go to a project Settings > General
• Expand and collapse some sections
• Add something to search bar
• Clear search
• expectation: The sections should be expanded / collapsed in the same way before you started the search.

Screenshots (strongly suggested)

It works 🎉

20210208_remember_expand_collapse

Does this MR meet the acceptance criteria?

Conformity

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