Freeze constants

.rubocop.yml

   Exclude:
     - 'app/models/session.rb'
 
-# Offense count: 3
-# Cop supports --auto-correct.
-Style/MutableConstant:
-  Exclude:
-    - 'app/models/participant.rb'
-    - 'app/models/token.rb'
-
 # Offense count: 1
 # Cop supports --auto-correct.
 # Configuration parameters: Strict.

app/models/participant.rb

     true => I18n.t('activities.state.present'),
     false => I18n.t('activities.state.absent'),
     nil => I18n.t('activities.state.unknown')
-  }
+  }.freeze
 
   # @return [String]
   #   the name for the Participant's current state in the current locale.
     true => 'ATTENDING',
     false => 'CANCELLED',
     nil => 'TENTATIVE'
-  }
+  }.freeze
 
   # @return [String]
   #   the ICal attending response.

app/models/token.rb

     password_reset: 'pw_reset',
     account_confirmation: 'confirm',
     api_authentication: 'api'
-  }
+  }.freeze
 
   validates :token, uniqueness: true, presence: true
   validates :user, presence: true