Add remove_concurrent_index to database helper

...te/20170216141440_drop_index_for_builds_project_status.rb

db/migrate/20170222143500_remove_old_project_id_columns.rb

db/migrate/20170313213916_add_index_to_user_ghost.rb

doc/development/migration_style_guide.md

...mplates/active_record/migration/create_table_migration.rb

generator_templates/active_record/migration/migration.rb

...or_templates/rails/post_deployment_migration/migration.rb

lib/gitlab/database/migration_helpers.rb

rubocop/cop/migration/add_concurrent_index.rb

rubocop/cop/migration/remove_concurrent_index.rb

rubocop/cop/migration/remove_index.rb

rubocop/rubocop.rb

spec/lib/gitlab/database/migration_helpers_spec.rb

spec/rubocop/cop/migration/remove_concurrent_index_spec.rb

spec/rubocop/cop/migration/remove_index_spec.rb