Limit access to the study group dashboard to teachers of that group

Also rename teacher_in_study_group? to add a `?` at the end

app/policies/application_policy.rb

@@ -41,10 +41,10 @@ class ApplicationPolicy
   end
   private :everyone_in_study_group
 
-  def teacher_in_study_group
+  def teacher_in_study_group?
     teacher? && everyone_in_study_group
   end
-  private :teacher_in_study_group
+  private :teacher_in_study_group?
 
   def initialize(user, record)
     @user = user

app/policies/exercise_policy.rb

@@ -3,10 +3,14 @@ class ExercisePolicy < AdminOrAuthorPolicy
     admin?
   end
 
-  [:show?, :study_group_dashboard?, :feedback?, :requests_for_comments?, :statistics?].each do |action|
+  [:show?, :feedback?, :requests_for_comments?, :statistics?].each do |action|
     define_method(action) { admin? || teacher? }
   end
 
+  def :study_group_dashboard?
+    admin? || teacher_in_study_group?
+  end
+
   [:clone?, :destroy?, :edit?, :update?, :export_external_check?, :export_external_confirm?].each do |action|
     define_method(action) { admin? || teacher_in_study_group || author? }
   end

app/policies/submission_policy.rb

@@ -14,6 +14,6 @@ class SubmissionPolicy < ApplicationPolicy
 
 
   def show_study_group?
-    admin? || teacher_in_study_group
+    admin? || teacher_in_study_group?
   end
 end