Add live dashboard for teachers in the context of an exercise

This commit also adds the fundamentals for ActionCable

app/policies/exercise_policy.rb

@@ -3,8 +3,8 @@ class ExercisePolicy < AdminOrAuthorPolicy
     admin?
   end
 
-  def show?
-    admin? || teacher?
+  [:show?, :study_group_dashboard?].each do |action|
+    define_method(action) { admin? || teacher? }
   end
 
   [:clone?, :destroy?, :edit?, :statistics?, :update?, :feedback?].each do |action|

app/policies/study_group_policy.rb

@@ -3,8 +3,8 @@ class StudyGroupPolicy < AdminOnlyPolicy
     admin? || teacher?
   end
 
-  [:show?, :destroy?, :edit?, :update?].each do |action|
-    define_method(action) { admin? || @user.teacher? && @record.users.include?(@user) }
+  [:show?, :destroy?, :edit?, :update?, :stream_la?].each do |action|
+    define_method(action) { admin? || @user.teacher? && @record.present? && @record.users.include?(@user) }
   end
 
   class Scope < Scope