Enable Rails/Validation Rubocop cop

Cop checks for the use of old-style attribute validation. Supports autocorrection which was run
to fix the files concerned

Closes #1414

.rubocop_todo.yml

@@ -141,23 +141,6 @@ Rails/TimeZone:
     - 'spec/models/campaign_spec.rb'
     - 'spec/models/conference_spec.rb'
 
-# Offense count: 16
-# Configuration parameters: Include.
-# Include: app/models/**/*.rb
-Rails/Validation:
-  Exclude:
-    - 'app/models/call_for_paper.rb'
-    - 'app/models/comment.rb'
-    - 'app/models/conference.rb'
-    - 'app/models/photo.rb'
-    - 'app/models/registration.rb'
-    - 'app/models/sponsor.rb'
-    - 'app/models/sponsorship_level.rb'
-    - 'app/models/subscription.rb'
-    - 'app/models/ticket.rb'
-    - 'app/models/ticket_purchase.rb'
-    - 'app/models/vposition.rb'
-
 # Offense count: 18
 Style/AccessorMethodName:
   Exclude: