DEV: Automatically require 'rails_helper' in all specs (#16077)

It's very easy to forget to add `require 'rails_helper'` at the top of every core/plugin spec file, and omissions can cause some very confusing/sporadic errors.

By setting this flag in `.rspec`, we can remove the need for `require 'rails_helper'` entirely.

spec/lib/backup_restore/backup_file_handler_multisite_spec.rb

@@ -1,6 +1,5 @@
 # frozen_string_literal: true
 
-require "rails_helper"
 require_relative "shared_context_for_backup_restore"
 
 describe BackupRestore::BackupFileHandler, type: :multisite do

spec/lib/backup_restore/backup_file_handler_spec.rb

@@ -1,6 +1,5 @@
 # frozen_string_literal: true
 
-require 'rails_helper'
 require_relative 'shared_context_for_backup_restore'
 
 describe BackupRestore::BackupFileHandler do

spec/lib/backup_restore/backuper_spec.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require 'rails_helper'
-
 describe BackupRestore::Backuper do
   it 'returns a non-empty parameterized title when site title contains unicode' do
     SiteSetting.title = 'Ɣ'

spec/lib/backup_restore/database_restorer_multisite_spec.rb

@@ -1,6 +1,5 @@
 # frozen_string_literal: true
 
-require "rails_helper"
 require_relative "shared_context_for_backup_restore"
 
 describe BackupRestore::DatabaseRestorer, type: :multisite do

spec/lib/backup_restore/database_restorer_spec.rb

@@ -1,6 +1,5 @@
 # frozen_string_literal: true
 
-require 'rails_helper'
 require_relative 'shared_context_for_backup_restore'
 
 describe BackupRestore::DatabaseRestorer do

spec/lib/backup_restore/local_backup_store_spec.rb

@@ -1,6 +1,5 @@
 # frozen_string_literal: true
 
-require 'rails_helper'
 require 'backup_restore/local_backup_store'
 require_relative 'shared_examples_for_backup_store'
 

spec/lib/backup_restore/meta_data_handler_spec.rb

@@ -1,6 +1,5 @@
 # frozen_string_literal: true
 
-require 'rails_helper'
 require_relative 'shared_context_for_backup_restore'
 
 describe BackupRestore::MetaDataHandler do

spec/lib/backup_restore/s3_backup_store_spec.rb

@@ -1,6 +1,5 @@
 # frozen_string_literal: true
 
-require 'rails_helper'
 require 's3_helper'
 require 'backup_restore/s3_backup_store'
 require_relative 'shared_examples_for_backup_store'

spec/lib/backup_restore/system_interface_multisite_spec.rb

@@ -1,6 +1,5 @@
 # frozen_string_literal: true
 
-require "rails_helper"
 require_relative "shared_context_for_backup_restore"
 
 describe BackupRestore::SystemInterface, type: :multisite do

spec/lib/backup_restore/system_interface_spec.rb

@@ -1,6 +1,5 @@
 # frozen_string_literal: true
 
-require 'rails_helper'
 require_relative 'shared_context_for_backup_restore'
 
 describe BackupRestore::SystemInterface do

spec/lib/backup_restore/uploads_restorer_spec.rb

@@ -1,7 +1,6 @@
 # frozen_string_literal: true
 # rubocop:disable Discourse/OnlyTopLevelMultisiteSpecs
 
-require 'rails_helper'
 require_relative 'shared_context_for_backup_restore'
 
 describe BackupRestore::UploadsRestorer do