Added Rubocop config for background migrations

This adds a Rubocop configuration file specific to
lib/gitlab/background_migrations. This configuration will be used to
(hopefully) make reviewing background migrations easier by enforcing
stricter rules compared to the rest of GitLab. Because this
configuration is directory specific it will only affect background
migrations.

lib/gitlab/background_migration/.rubocop.yml

+# For background migrations we define a custom set of rules to make it less
+# difficult to review these migrations. To reduce the complexity of these
+# migrations some rules may be stricter than the defaults set in the root
+# .rubocop.yml file.
+---
+inherit_from: ../../../.rubocop.yml
+
+Metrics/AbcSize:
+  Enabled: true
+  Max: 30
+  Details: >
+    Code that involves a lot of branches can be very hard to wrap your head
+    around.
+
+Metrics/PerceivedComplexity:
+  Enabled: true
+
+Metrics/LineLength:
+  Enabled: true
+  Details: >
+    Long lines are very hard to read and make it more difficult to review
+    changes.
+
+Metrics/MethodLength:
+  Enabled: true
+  Max: 30
+  Details: >
+    Long methods can be very hard to review. Consider splitting this method up
+    into separate methods.
+
+Metrics/ClassLength:
+  Enabled: true
+  Details: >
+    Long classes can be very hard to review. Consider splitting this class up
+    into multiple classes.
+
+Metrics/BlockLength:
+  Enabled: true
+  Details: >
+    Long blocks can be hard to read. Consider splitting the code into separate
+    methods.
+
+Style/Documentation:
+  Enabled: true
+  Details: >
+    Adding documentation makes it easier to figure out what a migration is
+    supposed to do.
+
+Style/FrozenStringLiteralComment:
+  Enabled: true
+  Details: >-
+    This removes the need for calling "freeze", reducing noise in the code.

lib/gitlab/background_migration/create_fork_network_memberships_range.rb

+# frozen_string_literal: true
+# rubocop:disable Metrics/LineLength
+# rubocop:disable Style/Documentation
+
 module Gitlab
   module BackgroundMigration
     class CreateForkNetworkMembershipsRange

lib/gitlab/background_migration/create_gpg_key_subkeys_from_gpg_keys.rb

+# frozen_string_literal: true
+# rubocop:disable Metrics/LineLength
+# rubocop:disable Style/Documentation
+
 class Gitlab::BackgroundMigration::CreateGpgKeySubkeysFromGpgKeys
   class GpgKey < ActiveRecord::Base
     self.table_name = 'gpg_keys'

lib/gitlab/background_migration/delete_conflicting_redirect_routes_range.rb

+# frozen_string_literal: true
+# rubocop:disable Metrics/LineLength
+# rubocop:disable Style/Documentation
+
 module Gitlab
   module BackgroundMigration
     class DeleteConflictingRedirectRoutesRange

lib/gitlab/background_migration/deserialize_merge_request_diffs_and_commits.rb

+# frozen_string_literal: true
+# rubocop:disable Metrics/MethodLength
+# rubocop:disable Metrics/LineLength
+# rubocop:disable Metrics/AbcSize
+# rubocop:disable Style/Documentation
+
 module Gitlab
   module BackgroundMigration
     class DeserializeMergeRequestDiffsAndCommits

lib/gitlab/background_migration/migrate_build_stage_id_reference.rb

+# frozen_string_literal: true
+# rubocop:disable Style/Documentation
+
 module Gitlab
   module BackgroundMigration
     class MigrateBuildStageIdReference

lib/gitlab/background_migration/migrate_events_to_push_event_payloads.rb

+# frozen_string_literal: true
+# rubocop:disable Metrics/LineLength
+# rubocop:disable Style/Documentation
+
 module Gitlab
   module BackgroundMigration
     # Class that migrates events for the new push event payloads setup. All

lib/gitlab/background_migration/migrate_stage_status.rb

+# frozen_string_literal: true
+# rubocop:disable Metrics/AbcSize
+# rubocop:disable Style/Documentation
+
 module Gitlab
   module BackgroundMigration
     class MigrateStageStatus

lib/gitlab/background_migration/migrate_system_uploads_to_new_folder.rb

+# frozen_string_literal: true
+# rubocop:disable Metrics/LineLength
+# rubocop:disable Style/Documentation
+
 module Gitlab
   module BackgroundMigration
     class MigrateSystemUploadsToNewFolder

lib/gitlab/background_migration/move_personal_snippet_files.rb

+# frozen_string_literal: true
+# rubocop:disable Metrics/LineLength
+# rubocop:disable Style/Documentation
+
 module Gitlab
   module BackgroundMigration
     class MovePersonalSnippetFiles

lib/gitlab/background_migration/normalize_ldap_extern_uids_range.rb

+# frozen_string_literal: true
+# rubocop:disable Metrics/MethodLength
+# rubocop:disable Metrics/LineLength
+# rubocop:disable Metrics/ClassLength
+# rubocop:disable Metrics/BlockLength
+# rubocop:disable Style/Documentation
+
 module Gitlab
   module BackgroundMigration
     class NormalizeLdapExternUidsRange

lib/gitlab/background_migration/populate_fork_networks_range.rb

+# frozen_string_literal: true
+# rubocop:disable Metrics/MethodLength
+# rubocop:disable Metrics/LineLength
+# rubocop:disable Style/Documentation
+
 module Gitlab
   module BackgroundMigration
     class PopulateForkNetworksRange

lib/gitlab/background_migration/populate_merge_requests_latest_merge_request_diff_id.rb

+# frozen_string_literal: true
+# rubocop:disable Style/Documentation
+
 module Gitlab
   module BackgroundMigration
     class PopulateMergeRequestsLatestMergeRequestDiffId