Implement offline POST method for starting an offline participation

app/policies/contest_policy.rb

@@ -47,24 +47,28 @@ class ContestPolicy < ApplicationPolicy
     record.user.id == user.id || user.admin?
   end
 
+  def offline?
+    record.offline_form
+  end
+
   def offline_new?
-    true
+    offline?
   end
 
   def offline_create?
-    true
+    offline?
   end
 
   def offline_edit?
-    true
+    offline?
   end
 
   def offline_update?
-    true
+    offline?
   end
 
   def scoreboard?
-    true
+    record.public
   end
 
   def upload_csv?