Extended Exercises by worktime, difficulty and tags, added ProxyExercises as prework for recommendations

Tags can be added to exercises in the edit view. Tags can monitored under /tags.
Added the concept of ProxyExercises which are a collection of Exercises. They can be found under /proxy_exercises
Added Interventions as prework to show interventions later to the user.
Added exercise/[:id]/working_time to return the working time of the user in this exercise and the average working time of all users in this exercise

app/views/exercises/implement.html.slim

@@ -22,3 +22,4 @@
     #questions-column
       #questions-holder data-url="#{qa_url}/qa/index/#{@exercise.id}/#{@user_id}"
     = qa_js_tag
+