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

db/migrate/20170206152503_add_user_feedback.rb

@@ -0,0 +1,11 @@
+class AddUserFeedback < ActiveRecord::Migration
+  def change
+    create_table :user_exercise_feedbacks do |t|
+      t.belongs_to :exercise, null: false
+      t.belongs_to :user, polymorphic: true, null: false
+      t.integer :difficulty
+      t.integer :working_time_seconds
+      t.string :feedback_text
+    end
+  end
+end