Convert global_search_*_tab feature flags to application settings

Background

As a part of our initiative to reduce feature flags in the Global Search group, we will convert all the global_search__tab ops feature flag to the admin settings.

Plan

• Add search column in ApplicationSettings with jsonb accessors: !177265 (merged), !178292 (merged)
• Update routing for the search admin settings from /admin/application_settings/advanced_search to /admin/application_settings/search: !178694 (merged)
• Fix the feature flags usage in their corresponding CE or EE edition: !178888 (merged), !179028 (merged)
• Improve search navigation code !179282 (merged)
• Implement backend and frontend changes. This must be together with the backfill migration !179688 (merged)
• Update documentation. I separated this to reduce changes in the implementation !179991 (merged) !180356 (merged)
• [-] %17.11 or later: Remove feature flags YAML files and update the backfill migration as no-op. This should be after the required stop because the backfill migration must be completed before removing the feature flags. Follow-up #517274 (closed).

Reference: https://docs.gitlab.com/ee/development/avoiding_required_stops.html#planning-the-required-stop-milestone