Ensure all ApplicationSetting attributes have a definition file and vice-versa

What does this MR do and why?

• Introduce scripts/cells/ci-ensure-application-settings-have-definition-file.rb to check that
  • Each ApplicationSetting attributes have a corresponding config/application_setting_columns/*.yml file
  • Each config/application_setting_columns/*.yml file actually relate to an existing ApplicationSetting attribute
• Introduce a new ensure-application-settings-have-definition-file job that runs the above script when db/structure.sql or config/application_setting_columns/*.yml files are touched

The job is currently passing: https://gitlab.com/gitlab-org/gitlab/-/jobs/8024846988 🚀

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Related to #496758 (closed).