Add frozen_string_literal magic comment

re: https://www.rubydoc.info/gems/rubocop/RuboCop/Cop/Style/FrozenStringLiteralComment

app/helpers/admin/volunteers_helper.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Admin
   module VolunteersHelper
     def can_manage_volunteers?(conference)

app/helpers/application_helper.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module ApplicationHelper
   # Returns a string build from the start and end date of the given conference.
   #

app/helpers/conference_helper.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module ConferenceHelper
   # Return true if only call_for_papers or call_for_tracks or call_for_booths is open
   def one_call_open(*calls)

app/helpers/date_time_helper.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module DateTimeHelper
   ##
   # Includes functions related to date or time manipulations

app/helpers/events_helper.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module EventsHelper
   ##
   # Includes functions related to events

app/helpers/format_helper.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module FormatHelper
   ##
   # Includes functions related to formatting (like adding classes, colors)

app/helpers/paths_helper.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module PathsHelper
   ##
   # Includes functions related to links or redirects

app/helpers/users_helper.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module UsersHelper
   ##
   # Includes functions related to users

app/helpers/versions_helper.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module VersionsHelper
   ##
   # Groups functions related to change description