Add option to suppress feedback messages

This is used to dynamically exclude some test results from being shown to users, but still allows them to run in the background (e.g., for research).

app/models/code_ocean/file.rb

@@ -49,6 +49,7 @@ module CodeOcean
     validates :feedback_message, absence: true, unless: :teacher_defined_assessment?
     validates :hashed_content, if: :content_present?, presence: true
     validates :hidden, inclusion: [true, false]
+    validates :hidden_feedback, inclusion: [true, false]
     validates :name, presence: true
     validates :read_only, inclusion: [true, false]
     validates :role, inclusion: {in: ROLES}