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/models/javascript_cache_spec.rb

@@ -1,6 +1,4 @@
 # frozen_string_literal: true
-require 'rails_helper'
-
 RSpec.describe JavascriptCache, type: :model do
   let!(:theme) { Fabricate(:theme) }
   let(:theme_field) { ThemeField.create!(theme: theme, target_id: 0, name: "header", value: "<a>html</a>") }