Fix rubocop issues, introduce usage of frozen literal to improve performance

app/channels/application_cable/channel.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module ApplicationCable
   class Channel < ActionCable::Channel::Base
     protected

app/channels/application_cable/connection.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module ApplicationCable
   class Connection < ActionCable::Connection::Base
     identified_by :current_user

app/channels/hashtag_channel.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class HashtagChannel < ApplicationCable::Channel
   def subscribed
     tag = params[:tag].downcase

app/channels/public_channel.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class PublicChannel < ApplicationCable::Channel
   def subscribed
     stream_from 'timeline:public', lambda { |encoded_message|

app/channels/timeline_channel.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class TimelineChannel < ApplicationCable::Channel
   def subscribed
     stream_from "timeline:#{current_user.account_id}"