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/mailers/group_smtp_mailer_spec.rb

@@ -1,5 +1,4 @@
 # frozen_string_literal: true
-require 'rails_helper'
 require 'email/receiver'
 
 describe GroupSmtpMailer do

spec/mailers/invite_mailer_spec.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require "rails_helper"
-
 describe InviteMailer do
 
   describe "send_invite" do

spec/mailers/rejection_mailer_spec.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require "rails_helper"
-
 describe RejectionMailer do
 
   describe "send_rejection" do

spec/mailers/subscription_mailer_spec.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require "rails_helper"
-
 describe SubscriptionMailer do
   fab!(:user) { Fabricate(:user) }
 

spec/mailers/test_mailer_spec.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require "rails_helper"
-
 describe TestMailer do
 
   describe "send_test" do

spec/mailers/user_notifications_spec.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require "rails_helper"
-
 describe UserNotifications do
 
   let(:user) { Fabricate(:admin) }

spec/mailers/version_mailer_spec.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require "rails_helper"
-
 describe VersionMailer do
   subject { VersionMailer.send_notice }