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/models/concerns/user.rb

@@ -8,6 +8,10 @@ module User
     has_many :exercises, as: :user
     has_many :file_types, as: :user
     has_many :submissions, as: :user
+    has_many :user_proxy_exercise_exercises, as: :user
+    has_many :user_exercise_interventions, as: :user
+    has_many :interventions, through: :user_exercise_interventions
+
 
     scope :with_submissions, -> { where('id IN (SELECT user_id FROM submissions)') }
   end