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.

plugins/poll/spec/lib/poll_spec.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require 'rails_helper'
-
 describe DiscoursePoll::Poll do
   fab!(:user) { Fabricate(:user) }
   fab!(:user_2) { Fabricate(:user) }

plugins/poll/spec/lib/polls_updater_spec.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require 'rails_helper'
-
 describe DiscoursePoll::PollsUpdater do
 
   def update(post, polls)

plugins/poll/spec/lib/pretty_text_spec.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require 'rails_helper'
-
 describe PrettyText do
 
   def n(html)

plugins/poll/spec/models/poll_spec.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require 'rails_helper'
-
 describe ::DiscoursePoll::Poll do
   describe ".transform_for_user_field_override" do
     it "Transforms UserField name if a matching CustomUserField is present" do

plugins/poll/spec/serializers/poll_option_serializer_spec.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require 'rails_helper'
-
 def serialize_option(option, user)
   PollOptionSerializer.new(
     option,