Adjust teacher permissions:

* Hide non-public exercises and proxy exercises
* Hide average working time value on statistic page
* Add hint about filter to submission page for teachers
* Hide display name from feedback page

app/policies/proxy_exercise_policy.rb

@@ -4,7 +4,7 @@ class ProxyExercisePolicy < AdminOrAuthorPolicy
   end
 
   def show?
-    admin? || teacher?
+    admin? || teacher_in_study_group? || teacher? && @record.public? || author?
   end
 
   [:clone?, :destroy?, :edit?, :update?].each do |action|