Add search to user 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?

It adds the search settings feature to all the User settings’ pages.

Screenshots (strongly suggested)

How to test?

1. Enable the search_settings_in_page feature flag by executing rails c && Feature.enable(:search_settings_in_page)
2. Visit http://localhost:3000/-/profile

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team