Use plain SQL schema

changelogs/unreleased/ab-structuresql.yml

config/application.rb

danger/database/Dangerfile

doc/development/database_review.md

doc/development/omnibus.md

doc/development/scalability.md

doc/update/patch_versions.md

doc/update/upgrading_from_source.md

lib/gitlab/database/schema_cleaner.rb

lib/gitlab/fake_application_settings.rb

lib/tasks/gitlab/db.rake

scripts/schema_changed.sh

scripts/utils.sh

spec/fixtures/gitlab/database/structure_example.sql

spec/fixtures/gitlab/database/structure_example_cleaned.sql

spec/lib/gitlab/danger/helper_spec.rb

spec/lib/gitlab/database/schema_cleaner_spec.rb

spec/migrations/active_record/schema_spec.rb

spec/tasks/gitlab/backup_rake_spec.rb

spec/tasks/gitlab/db_rake_spec.rb