POC: Search general settings

What does this MR do?

It adds a search box on top of the Project’s General Settings page that allows to find content within the page’s sections. When content is found, results are displayed in the following way:

• Sections that don’t match the search term are hidden.
• Text that matches the search term is highlighted.

This feature is behind a feature flag named search_settings_in_page. To test locally, follow these steps:

cd path_to_gitlab_project
bin/rails console
Feature.enable(:search_settings_in_page)

Screenshots (strongly suggested)

settings-search-highlight

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

Related to #292941 (closed)