Replace global_search_*_tab feature flags with application settings

What does this MR do and why?

• Add a backfill migration.
• We are adding the global search settings for both CE and EE.
• Move menu item from EE to CE to access the settings page in CE as well.
• Use the admin setting instead of a ff.

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

• Simulate a CE instance with a licensed GDK: https://docs.gitlab.com/ee/development/ee_features.html#simulate-a-ce-instance-with-a-licensed-gdk

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Mode	Before	After
CE	The Page did not exist
EE

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Run the bin/rails db:migrate and notice that the data is backfilled in the table based on the configuration of the ff you have.
2. Open admin/application_settings/search url on GDK for both CE and EE mode and see the Global Search Settings Tab you can toggle the checkboxes and save them and notice the changes are being reflected in the UI.
3. The changes take some time to reflect so do a refresh and then go to /search
4. You will notice that the tabs will not be visible.
5. Also if you directly open the URL then also you will be redirected back to /search

Related to #511347 (closed)