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/file_store/base_store_spec.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require 'rails_helper'
-
 RSpec.describe FileStore::BaseStore do
   fab!(:upload) { Fabricate(:upload, id: 9999, sha1: Digest::SHA1.hexdigest('9999')) }
 

spec/lib/file_store/local_store_spec.rb

@@ -1,6 +1,5 @@
 # frozen_string_literal: true
 
-require 'rails_helper'
 require 'file_store/local_store'
 
 describe FileStore::LocalStore do

spec/lib/file_store/s3_store_spec.rb

@@ -1,6 +1,5 @@
 # frozen_string_literal: true
 
-require 'rails_helper'
 require 'file_store/s3_store'
 require 'file_store/local_store'