Enable frozen string literals across the project (#3483)

https://docs.rubocop.org/rubocop/cops_style.html#stylefrozenstringliteralcomment

This feature is designed to help transition from mutable string literals
to frozen string literals. The "frozen_string_literal: true" magic
comment was added to the top of all files to enable frozen string
literals. Frozen string literals may be default in future Ruby. Using
frozen string literals is good practice and easier to reason about.

Author: jdufresne

.rubocop.yml

@@ -168,9 +168,6 @@ Style/Encoding:
 Style/EndlessMethod:
   Enabled: true
 
-Style/FrozenStringLiteralComment:
-  Enabled: false
-
 Style/HashConversion:
   Enabled: true
 

Appraisals

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 appraise 'rails-6.0' do
   gem 'rails', '~> 6.0.0'
   gem 'sassc-rails', '~> 2.1'

Gemfile

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 source 'https://rubygems.org'
 
 gem 'appraisal', '>= 2.0'

Rakefile

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Add your own tasks in files placed in lib/tasks ending in .rake,
 # for example lib/tasks/capistrano.rake, and they will automatically be available to Rake.
 

app/controllers/rails_admin/application_controller.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'rails_admin/abstract_model'
 
 module RailsAdmin

app/controllers/rails_admin/main_controller.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RailsAdmin
   class MainController < RailsAdmin::ApplicationController
     include ActionView::Helpers::TextHelper

app/helpers/rails_admin/application_helper.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RailsAdmin
   module ApplicationHelper
     def authorized?(action_name, abstract_model = nil, object = nil)

app/helpers/rails_admin/form_builder.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'nested_form/builder_mixin'
 
 module RailsAdmin

app/helpers/rails_admin/main_helper.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RailsAdmin
   module MainHelper
     def rails_admin_form_for(*args, &block)

config/initializers/active_record_extensions.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 ActiveSupport.on_load(:active_record) do
   module ActiveRecord
     class Base