Implement working time graph for study group dashboard

(so far, without live update)

app/helpers/action_cable_helper.rb

@@ -1,8 +1,10 @@
 module ActionCableHelper
   def trigger_rfc_action_cable
+    # Context: RfC
     if submission.study_group_id.present?
       ActionCable.server.broadcast(
         "la_exercises_#{exercise_id}_channel_study_group_#{submission.study_group_id}",
+        type: :rfc,
         id: id,
         html: (ApplicationController.render(partial: 'request_for_comments/list_entry',
                                             locals: {request_for_comment: self})))
@@ -10,6 +12,19 @@ module ActionCableHelper
   end
 
   def trigger_rfc_action_cable_from_comment
+    # Context: Comment
     RequestForComment.find_by(submission: file.context).trigger_rfc_action_cable
   end
+
+  def trigger_working_times_action_cable
+    # Context: Submission
+    if study_group_id.present?
+      ActionCable.server.broadcast(
+          "la_exercises_#{exercise_id}_channel_study_group_#{study_group_id}",
+          type: :working_times,
+          working_time_data: exercise.get_working_times_for_study_group(study_group_id, user))
+    end
+  end
 end
+
+# TODO: Check if any user is connected and prevent preparing the data otherwise