Add search column to application settings

What does this MR do and why?

Add a new column search of the type jsonb in the application_settings table. This jsonb column can have six attributes (listed below) in ee and three (global_search_merge_requests_enabled and global_search_users_enabled global_search_work_items_enabled) in CE.

• global_search_code_enabled
• global_search_commits_enabled
• global_search_merge_requests_enabled
• global_search_wiki_enabled
• global_search_users_enabled
• global_search_snippet_titles_enabled
• global_search_work_items_enabled

This column can be used to show the settings in the admin panel and eventually, few global search ops feature flags can be deprecated.

All three attributes have the default value of true. In the follow-up MRs the backfilling of this column will happen.

References

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• Documentation Link related to adding application settings: https://docs.gitlab.com/ee/development/application_settings.html

MR acceptance checklist

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Before	After

How to set up and validate locally

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

• Check out the branch and run the DB migrations using bin/rails db:migrate
• Open the rails console

bin/rails c

• Check the existing ApplicationSetting record. The value of search should be {}

ApplicationSetting.last.search

• Check the new ApplicationSetting record. The value of search should be {"global_search_code_enabled"=>true, "global_search_commits_enabled"=>true, "global_search_merge_requests_enabled"=>true, "global_search_wiki_enabled"=>true, "global_search_users_enabled"=>true, "global_search_snippet_titles_enabled"=>true}

ApplicationSetting.new.search

Related to #511347 (closed)