Copy issues into a separate ES index

...rate/20201217132603_create_elastic_reindexing_subtasks.rb

db/schema_migrations/20201217132603

db/structure.sql

doc/integration/elasticsearch.md

...p/models/concerns/elastic/application_versioned_search.rb

ee/app/models/elastic/migration_record.rb

ee/app/models/elastic/reindexing_subtask.rb

ee/app/models/elastic/reindexing_task.rb

ee/app/services/elastic/cluster_reindexing_service.rb

ee/app/services/elastic/data_migration_service.rb

.../admin/application_settings/_elasticsearch_form.html.haml

ee/app/workers/concerns/elastic/migration_options.rb

ee/app/workers/concerns/elastic/migration_state.rb

ee/app/workers/elastic/migration_worker.rb

ee/app/workers/elastic_delete_project_worker.rb

ee/changelogs/unreleased/273264-copy-issues-to-new-index.yml

...igrate/20201123123400_migrate_issues_to_separate_index.rb

ee/lib/elastic/class_proxy_util.rb

ee/lib/elastic/instance_proxy_util.rb

ee/lib/elastic/latest/issue_config.rb