Add search box to settings project page

What does this MR do?

This merge request adds a search box to the top of the Project’s general settings page. It is part of the effort to implement in-page search as described in #292941 (closed).

The search box is hidden behind a feature flag and does not provide any functionality yet. If you want to test the whole functionality locally, you can do it in this MR: !50166 (merged).

If you just want to see how it looks in the Settings page, enable the feature flag following the next steps:

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

Screenshots (strongly suggested)

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)