setup and rubocop corrections

app/controllers/application_controller.rb

@@ -1,2 +1,4 @@
+# frozen_string_literal: true
+
 class ApplicationController < ActionController::API
 end

app/jobs/application_job.rb

@@ -1,2 +1,4 @@
+# frozen_string_literal: true
+
 class ApplicationJob < ActiveJob::Base
 end

app/mailers/application_mailer.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class ApplicationMailer < ActionMailer::Base
   default from: 'from@example.com'
   layout 'mailer'

app/models/application_record.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class ApplicationRecord < ActiveRecord::Base
   self.abstract_class = true
 end