Add live dashboard for teachers in the context of an exercise

This commit also adds the fundamentals for ActionCable

app/channels/application_cable/channel.rb

@@ -0,0 +1,4 @@
+module ApplicationCable
+  class Channel < ActionCable::Channel::Base
+  end
+end

app/channels/application_cable/connection.rb

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+module ApplicationCable
+  class Connection < ActionCable::Connection::Base
+    identified_by :current_user
+
+    def connect
+      self.current_user = find_verified_user
+    end
+
+    def disconnect
+      # Any cleanup work needed when the cable connection is cut.
+    end
+
+    private
+
+    def session
+      # `session` is not available here, so that we need to use `cookies.encrypted` instead
+      cookies.encrypted[Rails.application.config.session_options[:key]].symbolize_keys
+    end
+
+    def find_verified_user
+      # Finding the current_user is similar to the code used in application_controller.rb#current_user
+      current_user = ExternalUser.find_by(id: session[:external_user_id]) || InternalUser.find_by(id: session[:user_id])
+      if current_user
+        current_user
+      else
+        reject_unauthorized_connection
+      end
+    end
+  end
+end

app/channels/la_exercises_channel.rb

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+class LaExercisesChannel < ApplicationCable::Channel
+
+  def subscribed
+    stream_from specific_channel
+  end
+
+  def unsubscribed
+    stop_all_streams
+  end
+
+  private
+  def specific_channel
+    reject unless StudyGroupPolicy.new(current_user, StudyGroup.find_by(id: params[:study_group_id])).stream_la?
+    "la_exercises_#{params[:exercise_id]}_channel_study_group_#{params[:study_group_id]}"
+  end
+end